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doi:10.2204/iodp.proc.348.103.2015

Lithology

Evaluation of lithology was conducted on core samples from Holes C0002M (475–512.5 mbsf) and C0002P (2163–2217.5 mbsf), as well as observations on cuttings from Holes C0002N (875.5–2325.5 mbsf) and C0002P (1965.5–3058.5 mbsf). Based on the integration of geological, geophysical, geochemical, and LWD data available from the cores and cuttings, together with the results from previously drilled Holes C0002A (Expedition 314 Scientists, 2009), C0002B (Expedition 315 Scientists, 2009), C0002F, C0002H, C0002I, C0002K, and C0002L (Strasser et al., 2014b), we identified four lithologic units and seven subunits (Fig. F6; Table T5). Units III–V were previously defined based on the evaluation of cuttings from Hole C0002F (Strasser et al., 2014b).

Hole C0002M

Four cores were recovered in Hole C0002M between 475 and 512.5 mbsf, with an average core recovery of ~43.8% (see “Operations”). Based on comparisons with core descriptions from Holes C0002B (Expedition 315 Scientists, 2009), C0002K, and C0002L (Strasser et al., 2014b), the cored interval (475–512.5 mbsf) is situated in the previously defined lithologic Unit II.

Lithologic variation description

The dominant lithology in Hole C0002M is greenish gray silty claystone. Minor lithologies include thin interbeds of silty sandstone and siltstone (Fig. F7).

The silty claystone is typically homogeneous. No major sedimentary structures were identified, but locally horizontal to gently inclined plane-parallel laminae and incipient fissility were observed (Fig. F8). The silty interbeds are usually <5 cm thick and show normal grain size gradation, fining upward to silty claystone. Their bases are defined by sharp surfaces (locally erosive), and their tops are diffuse. Such features are typical of fine-grained turbidites.

A siliciclastic assemblage of clay, silt, and sandstone dominates with quartz and feldspar as main minerals. Minor grains include micas (biotite and muscovite), glauconite, volcanic glass, and an extensive diversity of dense minerals, based on smear slide observations (Figs. F9, F10; also see Site C0002 smear slides in “Core descriptions”). Lithic fragments are generally observed but are most abundant in the sandy siltstone. Carbonate is primarily present in the form of nannofossils, foraminifers, and silt-size anhedral calcite. Organic matter is seen throughout the cores and is locally observed as <2 mm nodules, commonly pyritized.

Mineralogical and geochemical analyses

X-ray diffraction (XRD) patterns show average total clay content of 51 wt%, quartz content of 26 wt%, and feldspar content of 20 wt% (Fig. F11; Table T6). Calcite abundance determined by XRD is generally present in low amounts of 3 wt% on average; however, three samples show relatively high contents of up to 15 wt%. A comparison of all XRD data including cuttings and core in Holes C0002B, C0002F, C0002K, C0002L, C0002H, and C0002M is shown in summary Figure F12.

X-ray fluorescence (XRF) analyses performed on 11 samples do not show specific trends (Table T7). Average values for this data set are

  • Na2O = 2.37 wt%,
  • MgO = 2.30 wt%,
  • Al2O3 = 16.57 wt%,
  • SiO2 = 63.63 wt%,
  • P2O5 = 0.14 wt%,
  • K2O = 3.13 wt%,
  • CaO = 4.10 wt%,
  • TiO2 = 0.72 wt%,
  • MnO = 0.08 wt%,
  • Fe2O3 = 6.39 wt%, and
  • Loss on ignition (LOI) = 7.8 wt%.

A comparison of all XRF data of cuttings and core in Holes C0002F, C0002K, C0002L, C0002H, and C0002M is shown in summary Figure F13.

Biological features

Calcareous nannofossils are abundant throughout the cored interval (Fig. F9). Foraminifers, diatoms, and fragmented siliceous bioclasts (including sponge spicules and radiolarians) occur in trace amounts. Bioturbation of moderate severity was observed throughout the sequence, but no particular trace fossil could be identified (Fig. F8). Mottled textures, small (millimeter scale) burrows filled with sandy material, and disturbed bedding are the most common bioturbation features. A few networks of burrows are well imaged in the X-ray computed tomography (XRCT) images.

Authigenic components

Few authigenic components were identified in the silty claystone. Pyrite is widely distributed through the cores, occurring as framboids, and in the above-noted <2 mm nodules of organic matter (Sections 348-C0002M-1R-2, 16 cm; 1R-2, 117 cm; and 4R-3, 86 cm) (Fig. F10). Pyrite precipitation was also observed along a 5.5 cm vein in interval 348-C0002M-1R-2, 11–16 cm.

Interpretation

Lithologic Unit II was previously interpreted as the lower forearc succession of the Kumano Basin, dominated by the hemipelagic mud of distal turbidites (Expedition 315 Scientists, 2009; Strasser et al., 2014b). The four cores recovered from Hole C0002M are in agreement with this interpretation. Strasser et al. (2014b) suggested the presence of two coarsening-upward packages of thin turbidites between 480 and 460 mbsf and from 460 mbsf to the top of Unit II. The more common occurrence of slightly coarser sand interbeds in the upper Hole C0002M core sections may correspond to the proposed lower cycle.

Holes C0002N and C0002P

The lithologic unit and subunit boundaries determined in cuttings from Holes C0002N and C0002P are defined primarily on the basis of variations in percent sandstone versus percent silty claystone (Fig. F14; Table T8). Figures F15, F16, F17, F18, and F19 show representative lithologies and mineralogy as seen in the cuttings.

In Holes C0002N and C0002P, three units and seven subunits were defined. Hole C0002N was drilled between 860.5 and 2330 mbsf, and Hole C0002P was drilled between 1965.5 and 3058.5 mbsf. Lithologic units and subunits in Hole C0002N are in agreement with the boundaries determined during Expedition 338 in Hole C0002F (Strasser et al., 2014b), with few minor offsets in specific boundary depths that can be explained by the mixing of cuttings due to the usage of the underreamer in Hole C0002F (Strasser et al., 2014b) and/or the steep bedding.

Unit III (lower part of Kumano forearc basin)

  • Interval: cuttings Samples 348-C0002N-3-SMW through 24-SMW
  • Depth: ~875.5–975.5 mbsf
  • Lithology: greenish gray silty claystone

Hole C0002N drilling began within Unit III below the 20 inch casing shoe (860.3 mbsf), with collection of cuttings samples starting at ~875.5 mbsf (Sample 348-C0002N-3-SMW). The base of Unit III was first defined in Hole C0002A by LWD data (Expedition 314 Scientists, 2009) and by core and seismic integration at 915.5 mbsf (Expedition 315 Scientists, 2009). In Hole C0002F (Strasser et al., 2014b), the lithologic boundary is at 1025 mbsf. In Hole C0002N, the lithologic boundary is at 975.5 mbsf (Sample 24-SMW), with the first occurrence of sand and changes in mineralogy (Fig. F14; Table T5). The LWD boundary is set at 915 mbsf (see “Logging”). The discrepancies between the lithologic boundaries observed during Expeditions 338 and 348 are most likely due to the usage of the underreamer during Expedition 338, the relative uncertainty of cuttings descriptions, and the steep bedding as detected in LWD data.

Between 875.5 and 915.5 mbsf (Samples 3-SMW through 12-SMW), cuttings consist of ~40% fragments of cement derived from the earlier well completion (Strasser et al., 2014b) (Fig. F14). The cement contamination decreases with depth and disappears at 965.5 mbsf (Sample 22-SMW). A more detailed description of cement contamination in cuttings is in “Physical properties” in Strasser et al. (2014b).

The lithology in the formation cuttings is greenish gray silty claystone (Fig. F15; Table T8; also see macroscopic descriptions in VCDSCAN in “Supplementary material”). Locally, trace amounts of very fine, loose sand occur, some of which could also be disaggregated cement pieces. The silty claystone is semiconsolidated (i.e., compacted, but mechanically weak). In terms of mineralogy, quartz is the dominant mineral, glauconite minerals are present, and fossils are absent to rare and some are pyritized (Fig. F17A).

Unit IV (upper accretionary prism)

  • Interval: cuttings Samples 348-C0002N-24-SMW through 175-SMW
  • Depth: 975.5–1665.5 mbsf
  • Lithology: dominant greenish gray silty claystone, minor sandstone

In Hole C0002N, the Unit III/IV boundary is defined by the first occurrence of sandstone at 975.5 mbsf. In Hole C0002B (Expedition 315 Scientists, 2009), the Unit III/IV boundary is defined in cores by a relatively abrupt change in structural style and a shift in lithology from condensed silty claystone above to underlying interbeds of silty claystone, siltstone, and very fine to fine sandstone. In Hole C0002F (Strasser et al., 2014b), the Unit III/IV boundary is also defined by the first occurrence of sandstone, albeit in very small amounts, at 1025.5 mbsf (Sample 338-C0002F-45-SMW). The difference between Holes C0002F and C0002N likely occurs because of simultaneous drilling by the bit and the underreamer tool during Expedition 338 (see “X-ray diffraction mineralogy” and “Operations” in Strasser et al., 2014b). Another explanation could be the steep bedding of the rock formation.

Within Unit IV, five subunits are defined on the basis of the occurrence and absence of sand and sandstone (Fig. F14; Table T1). This is in accordance with LWD data interpretations and is consistent with the boundaries defined during Expedition 338 (Strasser et al., 2014b). Increasing and decreasing sand content characterizes the following subunits:

  • Subunit IVA: 975.5–1045.5 mbsf (Samples 348-C0002N-24-SMW through 39-SMW).
  • Subunit IVB: 1045.5–1125.5 mbsf (Samples 39-SMW through 54-SMW).
  • Subunit IVC: 1125.5–1345.5 mbsf (Samples 54-SMW through 107-SMW).
  • Subunit IVD: 1345.5–1525.5 mbsf (Samples 107-SMW through 146-SMW).
  • Subunit IVE: 1525.5–1665.5 mbsf (Samples 146-SMW through 175-SMW).

In Subunit IVA, the dominant lithology is greenish gray silty claystone with sandstone as a minor lithology (Fig. F15; Table T8; also see macroscopic descriptions in VCDSCAN in “Supplementary material”). The silty claystone is semiconsolidated, and the cuttings shapes are subangular to angular. Sandstone cuttings are generally loose or very weakly consolidated (i.e., soft). Their typical shape is round to subangular. Loose quartz grains are the dominant component in the dispersed >63 µm sand-sized fraction.

The main mineralogy in Subunit IVA can be summarized as follows (Fig. F17A):

  • Quartz = dominant.
  • Feldspar = few.
  • Lithic fragments = few to common.
  • Mica = absent.
  • Volcanic glass = rare to common (but mostly as a few grains).
  • Pyrite = common.
  • Organics (including wood) = common.
  • Fossils = rare.

In Subunit IVB, the major lithology is greenish gray silty claystone. Only few grains of loose sand were detected in this subunit, in contrast to the description for Hole C0002F in which ~30% sand was observed (Strasser et al., 2014b). In Subunit IVC, the major lithology is greenish gray silty claystone (average = ~70%), and the minor lithology is loose sand (average = ~30%). In Subunit IVD, the major lithology is greenish gray silty claystone (average = ~65%). In Subunit IVE, the major lithology is greenish gray silty claystone showing a progressive increase in the amount of sand with depth. Mineralogy in Subunits IVB–IVE can be summarized as follows (Fig. F17A):

  • Pyrite decreases from few in Subunits IVB–IVD to rare in Subunit IVE.
  • Organic material/wood/lignite is common to locally abundant in Subunits IVB–IVD and decreases to few in Subunit IVE.
  • Fossils are rare in all subunits.
  • Glauconite is rare in Subunits IVB and IVC and is absent in Subunits IVD and IVE.

Examples of some of these minerals are shown in Figure F18.

Unit V (trench or Shikoku Basin hemipelagic deposits)

  • Intervals: cuttings Samples 348-C0002N-175-SMW through 327-SMW and 348-C0002P-9-SMW through 300-SMW
  • Depths: Hole C0002N = 1665.5–2330 mbsf; Hole C0002P = 1965.5–3058.5 mbsf
  • Lithology: dominant greenish gray silty claystone and fine silty claystone; minor fine sandstone

Unit V was drilled in Holes C0002N and C0002P. In Hole C0002N, the Unit IV/V boundary is defined by a lithologic change from sand and sandstone to silty claystone between 1655.5 and 1677.5 mbsf (Samples 348-C0002N-173-SMW through 177-SMW). In Hole C0002F (Strasser et al., 2014b), the Unit IV/V transition is a gradual decrease of sand, with its complete disappearance at the base of this interval.

In Hole C0002N, only Subunit VA was drilled within Unit V (Fig. F14; Table T5). Based on macroscopic observations of cuttings in Hole C0002N, Subunit VA is composed almost entirely of greenish gray silty claystone with a minor mineralogy of gray sandstone (Fig. F16; Table T8; also see macroscopic descriptions in VCDSCAN in “Supplementary material”). The silty claystone is semiconsolidated, and cuttings shape is subangular to angular. The >63 µm sand-sized fraction (Fig. F17A) contains quartz as the dominant mineral. Feldspar decreases from common to few with depth, lithic fragments are few, mica is rare to absent, volcanic glass is always rare, pyrite is common at the top of Subunit VA and then decreases to few, wood is mostly few and only locally common, and fossils are rare and become few at 1955.5 mbsf (Sample 274-SMW). Where present, fossils are commonly pyritized (Figs. F17A, F18). Glauconite is always rare.

Within Unit V in Hole C0002P, two subunits (VA and VB) were drilled. The subunits are defined on the basis of the occurrence of sandstone, silty claystone, and fine silty claystone observed in the cuttings (Fig. F14; Table T5):

  • Subunit VA: 1960.5–2625.5 mbsf (Samples 348-C0002P-9-SMW through 200-SMW).
  • Subunit VB: 2625.5–3058.5 mbsf (Samples 200-SMW through 300-SMW)

Between 1965.5 and 2015.5 mbsf (Samples 9-SMW through 28-SMW), cuttings consist of 30%–10% fragments of cement derived from sidetracking the well through the cemented casing. Cement contamination decreases with depth and disappears at 2025.5 mbsf (Sample 28-SMW). However, metal pieces from milling the casing persist in the samples downhole to 2195.5 mbsf (Sample 81-SMW).

In Hole C0002P, Unit V is dominated by greenish gray to medium gray silty claystone with minor amounts of sandstone (Figs. F14, F16; Table T8; also see macroscopic descriptions in VCDSCAN in “Supplementary material”). The silty claystone is locally firm to hard, and cuttings shapes are subangular to angular throughout. The sandstone is fine-grained and friable but locally also consolidated (cemented). Silty claystone alternates with several sandstone packages until 2625.5 mbsf. Below this depth, sandstone is observed in very small amounts. From 2360.5 mbsf, the fine-grained content increases in the silty claystone, providing a much finer texture. At 2625.5 mbsf, fine silty claystone is the dominant lithology. This depth marks the Subunit VA/VB boundary based on the observed change into a finer grained silty claystone and scarcity of sandstone interbeds. In Subunit VB, the fine silty claystone is sticky at certain depths, especially when wet (hygroscopic behavior). Few hard pieces of lithified fine silty claystone are observed throughout Unit V. These commonly exhibit polished internal shear surfaces with slickenlines and incipient fissility (see “Structural geology”).

The >63 µm sand-sized fraction (Fig. F17B) shows quartz as the dominant mineral, with feldspar decreasing from abundant to common/few with depth and few micas. Lithic fragments are rare until 2350 mbsf and are not observed below that depth. Volcanic glass is locally observed but always in small amounts (rare to few). Glauconite is rarely observed and was generally not detected below 2550 mbsf.

Smear slide observations

In Hole C0002N, 148 smear slides made from cuttings were examined at 10 m sampling intervals (Figs. F18, F19; also see Hole C0002N smear slides in SMEARSLD in “Supplementary material”). Quartz grains are common throughout the cuttings. Micas and heavy minerals are estimated to be few in Unit III and at certain depths within Subunit VA. Carbonate minerals are common in Unit III and Subunit IVC and are commonly observed in the upper part of Unit III, most likely because of cement contamination of the uppermost 100 m (20%–40% of Sample 348-C0002N-3-SMW to 1%–3% of Sample 24-SMW). Clay mineral clusters are dominant throughout the hole. Volcanic glass is generally scarce to common but is not observed in the lower part of Subunit VA (Samples 213-SMW through 241-SMW and 302-SMW through 327-SMW). Pyrite is commonly observed in Units III and IV and decreases slightly in Unit V. Glauconite is locally rare in Unit III and between Subunits IVD and VA. Sedimentary lithics are common throughout, and volcanic lithics are rarely observed in Unit V. Nannofossils, foraminifers, and diatoms are abundant in Unit III to Subunit IVC (Samples 5-SMW through 101-SMW) but are rarely seen in Subunits IVD–VA. Organic matter abundance generally decreases with depth.

In Hole C0002P, 115 smear slides were examined (Fig. F19; also see Hole C0002P smear slides in SMEARSLD in “Supplementary material”). Quartz is common to abundant throughout the drilled interval. Heavy minerals are rare to scarce at all depths. Carbonate minerals are common in Subunit VA but less so in Subunit VB. Clay minerals are usually abundant to dominant at all depths. Volcanic glass and volcanic lithics vary between few and common but are locally not observed. Pyrite is few to common at all depths. Sedimentary lithics are common to abundant. Organic matter is few to common throughout the hole.

Mineralogical and geochemical analyses

X-ray diffraction analysis

Bulk powder XRD results show the relative abundance of total clay minerals, quartz, feldspar, and calcite (Fig. F12; Table T9). Summary Figure F12 shows XRD data of cuttings from the 1–4 mm size fraction of Holes C0002F (Strasser et al., 2014b), C0002N, and C0002P, together with core data from Holes C0002B (Expedition 315 Scientists, 2009) and C0002H, C0002J, C0002K, and C0002L (Strasser et al., 2014b). In cuttings from Holes C0002N and C0002P, no significant differences in lithology are observed between the 1–4 and >4 mm cuttings size fractions; consequently, we use only cuttings from the 1–4 mm size fraction for further documentation, which is also in line with standard oil industry cuttings routines.

In Hole C0002N, XRD data were routinely measured starting at 875.5 mbsf (Sample 348-C0002N-3-SMW). The uppermost few measurements still show contamination by cement based on the relative high weight percentage of calcite, especially in the >4 mm size fraction. We observe a gradual increase between 875.5 and 1025.5 mbsf (Samples 3-SMW through 34-SMW) in total clay, quartz, and feldspar, as well as a large decrease in calcite from ~39 to 0.1 wt%. Because of this gradual change, together with the first occurrence of sandstone, the Unit III/IV boundary is defined at 975.5 mbsf (Sample 24-SMW). A corresponding boundary is defined in LWD data at ~915 mbsf (see “Logging”), whereas Expedition 315 observed an abrupt reduction in calcite content at the discordance at 922 mbsf (Expedition 315 Scientists, 2009). This shift was explained during Expedition 315 as an abrupt change of the depositional site from below (Unit IV) to above the carbonate compensation depth (Unit III). Similar but more gradual shifts in calcite content were also recorded at Ocean Drilling Program Sites 1175 and 1176 (Shipboard Scientific Party, 2001a, 2001b; Underwood et al., 2003).

In Hole C0002N, Unit IV shows two cycles of decreasing and then increasing total clay content downhole that correspond reasonably well to the subunit boundaries (Fig. F12). The amount of quartz (weight percent) increases slightly and then decreases in Subunits IVD and IVE. Feldspar shows similar changes as quartz throughout Unit IV. Calcite content determined by XRD generally remains low (~0.1 wt%) in Subunit IV, with a slight increase in Subunits IVC and IVD.

The Unit IV/V boundary at 1665.5 mbsf (Sample 175-SMW) is associated with a downhole increase in total clay content and a slight decrease in feldspar in Subunit VA. Quartz remains constant in Subunit VA, whereas calcite decreases from 2.3 to 0.1 wt% until 1885.5 mbsf (Sample 226-SMW), where it increases again to an average of ~4 wt% before decreasing at 1955.5 mbsf (Sample 239-SMW) to an average of ~0.1 wt%. Total clay and feldspar contents slightly decrease and then increase several times. Quartz content slightly increases and then decreases.

In Hole C0002P, cuttings samples were routinely analyzed starting at 1965.5 mbsf (Sample 348-C0002P-9-SMW) and overlap with Hole C0002N to 2325.5 mbsf (Sample 129-SMW). Total clay, quartz, feldspar, and calcite contents correlate well between Holes C0002P and C0002N in this overlap interval. Total clay starts decreasing from 2325.5 to 2425.5 mbsf (Sample 155-SMW), whereas both quartz and feldspar slightly increase. Between 2425.5 and 2705.5 mbsf (Sample 219-SMW), total clay content increases again. Quartz and feldspar contents remain approximately the same. Between 2705.5 and 2945.5 mbsf (Sample 273-SMW), total clay content decreases first and then starts to increase from 2825.5 mbsf (Sample 247-SMW), whereas quartz and feldspar have exactly the opposite trend compared with total clay. Calcite content still remains low at ~0.1 wt% throughout the whole depth range.

X-ray fluorescence

In order to characterize compositional trends in Holes C0002N and C0002P, XRF analysis was undertaken on cuttings samples (Fig. F13; Table T10). Major and minor element contents (SiO2, Al2O3, CaO, K2O, Na2O, Fe2O3, MgO, TiO2, P2O5, and MnO) were analyzed and complemented by LOI measurements. To compare the composition of cuttings sizes, both 1–4 and >4 mm cuttings size fractions were analyzed, with no significant differences, except for K2O (the >4 mm fraction shows a higher content than the 1–4 mm fraction) and MgO (the 1–4 mm fraction slightly exceeds the >4 mm fraction downhole to 1200 mbsf). Therefore, further analysis of Holes C0002N and C0002P only involved the 1–4 mm cuttings size fraction.

The compositional changes observed in the upper intervals of both Holes C0002N and C0002P are mainly due to the mixing of cement with formation. Based on CaO content, the lowest depth of cuttings contaminated by cement may be at ~955.5 mbsf in Hole C0002N (Sample 348-C0002N-20-SMW) and ~2045.5 mbsf in Hole C0002P (Sample 348-C0002P-32-SMW). Results for MnO are not shown because of the very small weight percentages. LOI within the zone of cement contamination shows 17.8 wt%, and such samples are not used in assessing averages.

In Hole C0002N below the cement contamination, CaO remains fairly constant (>3.0 wt%) throughout the profile. LOI and P2O5 decrease from Unit III to IV, whereas SiO2, Al2O3, K2O, Na2O, Fe2O3, MgO, and TiO2 increase. Most of the elements do not change significantly in Unit IV. Slight decreases in LOI and MgO are visible in Subunits IVD and IVE. LOI, Al2O3, Fe2O3, MgO, TiO2, and K2O decrease at the boundary between Units IV and V, and P2O5 and SiO2 increase. K2O increases throughout Unit V, whereas Na2O content slightly decreases. TiO2 increases slightly, but for the other elements the average values are about the same in Units IV and V. At the Subunit VA/VB boundary, there is a small offset in all the elements. In Subunit VA, MgO slightly increases and then decreases. K2O increases slightly, whereas the other elements seem constant. No significant changes are visible for most of the elements, except that there is a slight offset for Al2O3, Fe2O3, and MgO. The averages throughout Units IV and V are

  • MgO = 2.19 wt%,
  • Al2O3 = 16.06 wt%,
  • SiO2 = 65.44 wt%,
  • P2O5 = 0.09 wt%,
  • CaO = 3.03 wt%,
  • Fe2O3 = 5.44 wt%, and
  • LOI = 7.33 wt%.

In Unit IV, the averages are

  • K2O = 3.39 wt%,
  • TiO2 = 0.64 wt%, and
  • Na2O = 2.55 wt%.

In Unit V, the averages are

  • K2O = 3.71 wt%,
  • TiO2 = 0.67 wt%, and
  • Na2O = 2.30 wt%.

Hole C0002P only penetrates Unit V and overlaps with Hole C0002N from 1965.5 to 2330 mbsf. Along the overlapping interval, Na2O, MgO, Fe2O3, P2O5, and LOI correlate well with the data from Hole C0002N. However, for SiO2, K2O, Al2O3, TiO2, and CaO, there are offsets between 1965.5 and 2045.5 mbsf for data from Holes C0002N and C0002P, which result from the cement contamination mentioned above. SiO2, K2O, and Al2O3 remain relatively constant throughout Unit V at ~65, 3.5, and ~16 wt%, respectively. P2O5 and LOI similarly are invariable with depth (~0.07 and ~6 wt%, respectively). Fe2O3 and TiO2 contents are also fairly constant throughout (~5 and ~0.65 wt%, respectively), except for a cluster of samples between 2445.5 and 2525.5 mbsf (Samples 348-C0002P-159-SMW, 163-SMW, 168-SMW, 172-SMW, and 176-SMW), which are anomalously higher than the average. These values may be related to the presence of iron titanium–rich minerals. Na2O content averages ~2.3 wt% but is more scattered than other compositional elements, fluctuating between 2.1 and 2.4 wt%. K2O increases slowly with depth from 3.2 to 3.8 wt%, which may be related to smectite-illite transformation. MgO content generally decreases with depth from 2.1 to 1.9 wt% but exhibits a peak of higher values between 2585.5 and 2705.5 mbsf. A peak is also observed at the same depth interval for CaO content (maximum 2.4 wt% at 2665.5 mbsf; Sample 210-SMW). Apart from this interval, CaO content decreases with depth from ~1.8 to ~0.9 wt%.

Limitations of sediment cuttings analyses

In comparison to Expedition 338 (Strasser et al., 2014b), the cuttings data collected during this expedition correlate reasonably well with LWD and other data (see “Logging,” “Physical properties,” “Structural geology,” and “Geochemistry”), with depth shifts of ~10–40 m. However, specific lithologic variations that are normally observed and documented in cores cannot be recognized in cuttings.

An important limiting factor on the reliability of cuttings is the amount of their stratigraphic mixing. For example, the collapse of wall rock into the drilling mud (cavings) results in vertical mixing of lithologies, making it difficult to accurately reconstruct stratigraphic relationships. In Hole C0002N, several circumstances may have caused caving contamination, such as hole cleaning (1205–1221 mbsf), WOW (1662–1678 and 1992–2008 mbsf), and mud-loss treatment (2022–2038 mbsf). Sand was recovered in cuttings and drilling fluid as mostly unconsolidated material, and the >63 µm sand-sized fraction was separated during washing and sieving. Because of temperature, drilling mud circulation speed and viscosity, pH values, and chemical supplements added to the drilling mud, the lithified sediment is partly disaggregated. This makes it difficult to differentiate the drilling mud and disaggregated mud from mudstone or sand from sandstone.

In Holes C0002N and C0002P, defining units and subunits by the occurrence or disappearance of a different lithology (e.g., the appearance of sandstone) is the most reasonable approach. Because of slight smearing effects created by general circulation of cuttings fragments, the base of a unit or subunit can be defined only in a relatively imprecise way.

Hole C0002P cored interval

Six cores were taken in Hole C0002P, with an average recovery of 56.9% (minimum recovery of 4.2% for Core 348-C0002P-1R and maximum of 86.0% for Core 4R; see “Operations”). The cored interval (2163–2218.5 mbsf) was entirely within Unit V, as per the unit depths and descriptions established from Hole C0002N cuttings (Fig. F20A, F20B).

Lithologic variation

The main lithologies identified in the cored interval of Hole C0002P are greenish gray silty claystone and fine-grained sandstone (Figs. F20, F21). Minor lithologies include medium-gray fine silty claystone (Section 348-C0002P-6R-4) and black organic matter–rich interbeds and laminations. Bedding is very steep to vertical and generally planar but locally wavy. The silty claystones are firm to hard, have developed incipient fissility in places, and typically exhibit color banding, thin dark laminae, lenses, and irregular patches of sand. Mottled textures are locally observed, which may be related to possible postdepositional bioturbation. The sandstones are fine grained and friable but slightly cemented in places. They often show grain size gradation, fining into silty claystone. Sharp contact surfaces (locally erosive) between silty claystone and sandstone are common. This sediment is interpreted to be fine-grained turbidites.

A fault zone made up of highly fractured and deformed rock material (fault gouge) is identified in between intervals 348-C0002P-5R-4, 30–90 cm, and 5R-5, 0–30 cm (see “Structural geology”). Precipitation of calcite and other carbonate minerals occurred along localized veins (i.e., Section 5R-4, 58 cm). Slight lithologic changes are observed below this fault zone in both macro- and microscopic descriptions. The silty claystone is slightly sticky in places, and sandstone interbeds are less common.

Smear slides observation

Thirty-six smear slides were prepared and examined from the cored interval. A siliciclastic assemblage of clay, silt, and sand grains with quartz and feldspar dominates these lithologies. Clay minerals, micas (biotite and muscovite), diverse dense minerals, and lithic fragments are common components (Figs. F22, F23; also see SMEARSLD in “Supplementary material”).

Quartz is common to dominant at all depths. Feldspar and mica are few in all samples and rare or not observed in and near the fault zone. Heavy minerals are common in and close to Core 348-C0002P-3R. Carbonate minerals are rarely observed above the fault zone and are few below the fault zone, but there are calcite veins within the fault zone. Clay minerals are common to abundant. Volcanic glass is commonly observed in sandstone and below and in the fault zone. Pyrite is generally observed but is only rare in the fault zone. Sedimentary lithics vary from common to dominant, depending on the lithology. Volcanic lithics are generally observed below Core 4R-1. Pelagic grains including nannofossils, foraminifers, and diatoms are only observed in Cores 1R and 2R and below the fault zone. Organic matter is commonly observed in the black beds and laminations but is only rare in the fault zone.

Mineralogical and geochemical analyses

Bulk powder XRD results show the relative abundance of total clay minerals, quartz, plagioclase, and calcite throughout the core. The data are plotted in Figures F12 and F24 and in Table T11. Among the major minerals, total clay, quartz, and feldspar show some heterogeneity in the uppermost sections of the core. Total clay shows the greatest amount of variation throughout the core, ranging from ~58 to 62 wt%. Quartz, feldspar, and calcite do not show significant variations throughout the core. The fault zone at 2205 mbsf also shows no changes in sedimentary matrix mineralogy. Cuttings from Holes C0002N and C0002P show similar trends as the core. Interestingly, the cuttings from Hole C0002P have slightly lower total clay values and higher quartz values than the core and the cuttings from Hole C0002N. One possible reason could be the change in the drill mud composition.

XRF composition shows no major changes within the core, except a slight decrease of average SiO2 and CaO and a slight increase of average MgO and Al2O3 (Fig. F25; Table T12). No significant changes in the fault zone at 2205 mbsf are recognized. The overall trends of the XRF core composition are similar to those observed in cuttings from Holes C0002N and C0002P. However, the cuttings taken in the same hole as the core show consistently lower values in MgO, Al2O3, and Fe2O3 and higher values in SiO2, K2O, Ca2O, and LOI. One reason could again be a change in the drill mud composition.

Bulk elemental compositional variation across the fault zone in Sections 348-C0002P-5R-4, 35–91 cm, and 5R-5, 0–59 cm, was examined using XRF core scanning (Fig. F26). The fault zone shows significantly higher values of Fe2O3 and lower values of K2O between the lower half of Section 5R-4 and the upper half of Section 5R-5 (see “Structural geology”). The veins encountered in the fault zone are enriched in CaO and MnO.

Biological features

Calcareous nannofossils, foraminifers, and diatoms are observed in the uppermost sections of the core and below the fault zone. Organic matter is abundant in the black laminations, beds, and isolated specks but rare within the fault zone. Mild bioturbation were locally observed within the silty claystones, but no particular ichnotaxa were identified. Mottled textures and small burrows (millimeter-scale tubes) preserved by carbonate material (Fig. F21B) are the most common bioturbation features. Burrows are very clear in the XRCT images.

Authigenic components

Few authigenic components were identified in the silty claystones and sandstones. Pyrite was generally observed throughout the cores, occurring as framboids and in <2 mm nodules.

Comparison of core and cuttings

We compared rock composition and sedimentologic features from the core interval between 2163 and 2218.5 mbsf with cuttings (Samples 348-C0002P-71-SMW through SMW-86; Fig. F27). The main lithologies can be identified in both the cores and the cuttings; both show greenish gray silty claystone and fine-grained sandstone. In the cuttings, however, the sand fraction is often disaggregated and appears as loose sand. Firmer cuttings of sandstone are detected in the >4 mm size fractions (Fig. F27A). Minor lithologies like black organic matter–rich interbeds and laminations in the core can also be observed in the cuttings, as well as occasional sedimentary structures like bedding and bioturbation (Fig. F27B). Calcite veins that occur in Core 348-C0002P-5R-4 can be found in the form of small pieces in the cuttings. Although best observed in cores, shear features can also be recognized in cuttings. Common hard silty claystone cuttings exhibit polished shear surfaces with distinct slickenlines. Calcite vein pieces also show these striations (see “Structural geology”).

Mineralogical and geochemical comparison

X-ray diffractometry

XRD data from the core intervals of Holes C0002M and C0002P and cuttings from Holes C0002N and C0002P were compared with XRD data from Expeditions 338 (Holes C0002K, C0002L, C0002J, and C0002H; Strasser et al., 2014b) and 315 (Hole C0002B; Expedition 315 Scientists, 2009) (Fig. F12; Table T9). The data display the mineral variation with depth throughout all units identified at Site C0002.

Total clay content in cores between 200.9 and 506.5 mbsf (Holes C0002J, C0002K, C0002L, and C0002M) averages ~55 wt%, whereas total clay in Hole C0002B core has slightly lower average values of ~45 wt%. At the Unit III/IV boundary at ~915 mbsf, total clay increases to ~65 wt% in Hole C0002B core. This increase is also visible in core from Hole C0002H. The cuttings from Hole C0002N show similar higher average total clay values of ~60–65 wt%, which also correlate well with cuttings from Hole C0002F and cuttings and core data from Hole C0002P. At the Subunit IVC/IVD boundary, total clay decreases slightly to an average of ~58–60 wt% and stays almost constant to the bottom of the hole at 3058.5 mbsf. Samples taken from Hole C0002P core show a wider range of total clay content, but their average is consistent with cuttings values from Holes C0002N and C0002P.

The average content of quartz in Hole C0002J, C0002K, C0002L, and C0002M cores is ~30 wt% and decreases at the Unit III/IV boundary to ~18 wt% in Hole C0002B core. In Units IV and V, quartz increases slightly with depth to ~30 wt%. Similarly, feldspar increases slightly from an average content of 18 wt% in Hole C0002J, C0002K, C0002L, and C0002M cores to ~25 wt% in Hole C0002B core and decreases at the Unit III/IV boundary back to ~18 wt% until the end of the hole at 3058.5 mbsf. Hole C0002J, C0002K, C0002L, C0002M, and C0002B cores show highly scattered calcite content between 0.1 and 30 wt%. This is similar to observations made in Hole C0002B (Expedition 315 Scientists, 2009), where calcite abundance ranges from 0.63 (trace) to 27.16 wt% with an average of 14.21 wt%. Apart from these values, calcite remains at ~0.1–2 wt% with no significant change between data from core and cuttings. Exceptional areas are those data with higher values from possible cement contamination. The XRD cuttings data are relatively homogeneous compared with the core data because of the preferential preservation of the fine-grained (more consolidated) sediment in the silty claystone (1–4 mm size fraction) with respect to coarse-grained (less consolidated) sandy sediment.

There are weight percent differences between calcite analyzed by XRD and calcium carbonate from coulometric analysis (see “Geochemistry” for carbonate data). Although XRD results show trace values of calcite, the carbonate coulometry data are consistently higher. This is probably because the normalization procedure used in the XRD analyses usually underestimates mineral content when it is <5%. In addition, XRD analyses only provide relative abundances by using singular value decomposition computations, considering only total clay minerals, quartz, feldspar, and calcite.

X-ray fluorescence

XRF data from the cored intervals of Holes C0002M and C0002P and cuttings of Holes C0002N and C0002P are compared with XRF data from Expedition 338 (Holes C0002K, C0002L, C0002J, C0002H; Strasser et al., 2014b) (Fig. F13; Table T10). The data display mineral variations with depth throughout all units identified at Site C0002.

SiO2 content remains relatively constant at ~65–70 wt% throughout all drilled sections at Site C0002. Al2O3 exhibits the same trend with depth, staying between ~15 and 18 wt%. The cores in shallower Holes C0002K, C0002L, C0002J, and C0002M show an average of 3 wt% for K2O. Analysis of deeper cuttings from Holes C0002F, C0002N, and C0002P reveal a slight increase of K2O from 3.1 to 3.6 wt%, which may be related to a beginning smectite-illite transformation. Hole C0002J, C0002K, and C0002P cores show slightly lower average K2O values but exhibit a similar increasing trend with depth. Average MgO values in the upper cores are highly scattered between 1 and 3 wt%. MgO contents in cuttings decrease with depth from ~2.5 to ~1.8 wt%. Hole C0002J and C0002H cores show significantly lower average values than cuttings from greater depth, whereas Hole C0002P core data are mainly in agreement with the cuttings. Na2O values for the upper cores average ~2.5 wt%. Cuttings data decrease in Na2O content to ~2000 mbsf and remain constant at ~2.1 wt% below that depth. Hole C0002H core data show lower values. Scattering between 0.4 and 0.85 wt% is observed in the upper cores for TiO2. The cuttings show a slight decrease in TiO2 in Unit IV from 0.7 to 0.6 wt% and then remain constant throughout Unit V. There are two areas of anomalous TiO2 content at ~2000 mbsf (average = 0.5 wt%) and ~2500 mbsf (average = 0.88 wt%). Hole C0002P core data scatter between 0.2 and 0.75 wt%. The upper cores and Hole C0002J core show highly scattered values of CaO content between 0.1 and 20 wt%. Apart from these scattered values, CaO remains at ~0.1 to 2 wt% with no significant change between data from core and cuttings, except in areas with cement contamination.

Interpretation of drilled stratigraphy

Unit III, consisting of silty claystone with trace amounts of sandy material, is interpreted as the fill of the lowermost part of the Kumano forearc basin and potentially prism slope basins (Expedition 315 Scientists, 2009). The composition of detrital grains is consistent with sediment supply from erosion of the 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).

Expedition 315 Scientists interpreted Unit III as forearc or supra-accretionary prism slope deposits that accumulated above the carbonate compensation depth, both prior to and during the early stages of formation of the Kumano Basin (Expedition 315 Scientists, 2009). Sediment-starved conditions were accompanied by a diverse assemblage of infauna. Local cementation of the sediment surface (by glauconite, possibly with phosphates and carbonates) was favored by slow sediment accumulation rates and exposure to oxygenated seawater (Strasser et al., 2014b). The base of Unit III was proposed to be a depositional contact between accreted trench-wedge sediment (Unit IV) and the initial deposits of hemipelagic silty claystone on the lowermost trench slope (Unit III) (Expedition 315 Scientists, 2009).

Seismic reflection profiles show complicated geometries with angular discordances and contrasts in structural style across the boundary. Expedition 315 Scientists interpreted the pronounced unconformity at ~922 mbsf as a manifestation of uplift along a system of out-of-sequence (splay) faults that occurred at ~5 Ma (Expedition 315 Scientists, 2009). Whether the uplift triggered erosion of accreted strata or favored slow sediment accumulation above the prism cannot be resolved without higher resolution biostratigraphy.

Unit IV represents the uppermost part of the older accretionary prism sediment, with silty claystone as the major lithology. Sandstone consists of mainly quartzo-feldspatic material, including common heavy-mineral assemblages and volcanic glass and variable but generally small amounts of organic/woody material. This assemblage is consistent with proximity to a volcanic source (Strasser et al., 2014b).

During Expedition 315, the depositional environment of Unit IV was 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 (Expedition 315 Scientists, 2009).

Unit IV consists of the most sandstone rich deposits recovered in Holes C0002F and C0002N. The most likely depositional environment is that of older accretionary prism slope basin fill or accreted submarine-fan deposits that accumulated in either a paleotrench or the Shikoku Basin.

The Unit IV/V boundary is set at 1665.5 mbsf. XRD and XRF analyses show a significant shift in mineralogy and element oxides at this interface, similar to what was identified during Expedition 338 in Hole C0002F (Strasser et al., 2014b).

Unit V consists essentially of silty claystone, the finest grained deposits within any unit in Holes C0002F and C0002N, and is also associated with the highest gamma radiation values (see “Logging”). Late Miocene age and sedimentology suggest that it is a candidate correlative unit to the lower hemipelagic Unit III drilled at subduction inputs Sites C0011 and C0012 (Expedition 322 Scientists, 2010a, 2010b), albeit possibly internally thrust duplicated and folded.