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doi:10.2204/iodp.proc.338.103.2014 LithologyHole C0002FBased on integration of data available from cuttings and LWD, we identified three lithologic units and five subunits in Hole C0002F (Fig. F18; Table T6), differentiated using geological, geophysical, and geochemical characteristics, as described in “Lithology” in the “Methods” chapter (Strasser et al., 2014a). The lithologic unit and subunit boundaries are defined primarily using the percent sandstone versus percent silty claystone supplemented by the quartz index (Q-index) (Figs. F18, F19, F20, F21, F22; Tables T6, T7, T8). Figures F23, F24, F25, and F26 show representative lithologies and rock components as seen in rock chips and smear slides. Between ~875.5 and 890.5 mbsf (Samples 338-C0002F-1-SMW though 14-SMW), cuttings consisted of 100% fragments of cement derived from the earlier well completion (Expedition 326) and no lithology was observed (Fig. F18). The first observations of formation in cuttings were present at 890.5 mbsf (Sample 338-C0002F-14-SMW). However, although formation cuttings were present from 890.5 to 930.5 mbsf (Samples 14-SMW through 22-SMW), cement remained the dominant constituent in the cuttings mix. From 930.5 to 965.5 mbsf (Samples 22-SMW through 30-SMW), there is a progressive increase in the proportion of formation relative to cement, up to 100% silty claystone. See “Physical properties” for a more detailed description of cement contamination and cuttings. Lithologic Unit III (lower part of Kumano forearc basin)
Hole C0002F drilling began within lithologic Unit III below the previously cemented 20 inch casing shoe at ~860 mbsf (Sample 338-C0002F-1-SMW). The base of lithologic Unit III was previously defined at 918.5 mbsf from LWD data (see “Logging while drilling”) and by core and seismic integration in Hole C0002B (Expedition 315 Scientists, 2009b; including detailed descriptions and interpretations). The lithologic boundary in Hole C0002F is identified at 1025.5 mbsf (Sample 45-SMW) (Table T6) with the first occurrence of sand and changes in mineralogy. Within the upper part of lithologic Unit III in Hole C0002F, ~70%–90% of the sampled cuttings consist of cement. As previously mentioned, samples contain 100% cement above 890.5 mbsf (Sample 14-SMW), which is consistent with other shipboard data (e.g., see “Physical properties” and discussion about mixing of cement). In cuttings from the formation, the lithology is greenish gray silty claystone (Figs. F18, F19). Locally, trace amounts of loose sand occur, some of which could also be disaggregated cement pieces. The silty claystone is semi-indurated (compact, but mechanically weak). In terms of accessory mineralogy (Fig. F20), glauconite grains are present (Fig. F23B–F23D) and fossils are absent to rare and some are pyritized (Fig. F25F). Lithologic Unit IV (upper accretionary prism)
In Hole C0002B (Expedition 315 Scientists, 2009b), the lithologic Unit III/IV boundary is defined by an abrupt change in structural style and a shift in lithology from condensed silty claystone above to underlying interbeds of silty claystone, siltstone, and sandstone. In Hole C0002F, the lithologic Unit III/IV boundary is defined by the first occurrence of sandstone, albeit in very small amounts at 1025.5 mbsf (Sample 338-C0002F-45-SMW). In general, the macroscopic observation of cuttings was difficult because of the mixing of cuttings caused by the underreamer (see “Lithologic Unit III (lower part of Kumano forearc basin)” and “Physical properties”). Based on calcite mineralogy analyzed by X-ray diffraction (XRD), the boundary is smeared by ~50–70 m because of simultaneous cutting by the bit and the underreamer (see “X-ray diffraction mineralogy” and “Operations”). Within lithologic Unit IV, five subunits are defined on the basis of the occurrence of sandstone (Fig. F18; Table T6). These subunits are characterized by increasing and decreasing sand content:
The dominant lithology in the subunits is greenish gray silty claystone with sandstone as a minor lithology (Figs. F18, F19). The silty claystone is semi-indurated, and the cuttings shape is subangular to angular (Table T7). Sandstone cuttings are generally loose or very weakly indurated (i.e., soft). Their typical shape is rounded. Loose quartz grains are the dominant component in the dispersed >63 µm sand-size fraction. In lithologic Unit IV, the Q-index shows overall increased grain sizes compared with the surrounding lithologic Units III and V, ranging from ~700 to 1800 µm in diameter (Fig. F22). At 1485.5 mbsf (Sample 338-C0002F-148-SMW), the Q-index shifts to higher values, with an average of ~1300 µm, and also shows greater fluctuations when compared with surrounding units. The main mineralogy in lithologic Subunit IVA can be summarized as follows (Fig. F20):
Smear slides show the high-temperature metamorphic mineral corundum at 1125.5 mbsf (Sample 338-C0002F-66-SMW) (Fig. F25A, F25B). Corundum is characteristic of contact-metamorphism of limestones and metamorphosed shales (e.g., schists). In lithologic Subunit IVA, the Q-index increases then decreases, in general showing relatively small grain sizes between 200 and 1200 µm (Fig. F22). In lithologic Subunit IVB (1140.5–1270.5 mbsf; Samples 338-C0002F-71-SMW to 100-SMW), the major lithology is greenish gray silty claystone (average ~70%). In lithologic Subunit IVC (1270.5–1420.5 mbsf; Samples 100-SMW to 134-SMW), the major lithology is greenish gray silty claystone (average ~70%). In lithologic Subunit IVD (1420.5–1600.5 mbsf; Samples 134-SMW to 182-SMW), the major lithology is greenish gray silty claystone (average ~65%). In lithologic Subunit IVE (1600.5–1740.5 mbsf; Samples 182-SMW to 215-SMW), the major lithology is greenish gray silty claystone, showing a progressive increase in amount with depth (average ~70%). In comparison to the overlying units and subunits, the sandstone in lithologic Subunit IVE appears to be more indurated. The mineralogy of lithologic Subunits IVB–IVE for the >63 µm sieved size fraction can be summarized as follows (Fig. F20; see Site C0002 smear slides in “Core descriptions”):
Examples of some of these minerals are shown in Figures F23, F24, F25, and F26. Lithologic Subunit IVD locally contains high organic matter content (1535.5 mbsf; Sample 338-C0002F-161-SMW). Lithologic Unit V (trench or Shikoku Basin hemipelagic deposits)
In Hole C0002F, the lithologic Unit IV/V boundary shows a gradual decrease of sand between 1680.5 and 1740.5 mbsf (Samples 338-C0002F-202-SMW through 214-SMW), with the complete disappearance of sandstone at the base of this interval (Figs. F18, F19). Lithologic Unit V is composed almost entirely of greenish gray silty claystone. The silty claystone is semi-indurated, and cuttings shape is subrounded to angular. The >63 µm sand-size fraction (Fig. F20) shows 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 lithologic Unit V 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 (Fig. F25F). Glauconite is always rare. The Q-index in lithologic Unit V shows overall increased grain sizes compared with the surrounding lithologic Units IV and V, ranging from ~500 to 1800 µm in diameter (Fig. F22). At 1485.5 mbsf (Sample 338-C0002F-148-SMW), the Q-index shifts to both higher values (average ~1300 µm) and greater fluctuations. Although the Q-index in lithologic Unit V shows the lowest values (250–950 µm) compared with lithologic Unit IV, it also suggests that some very fine sandstone layers may be present. Limitations using sediment cuttingsEven though the cuttings data correlate reasonably well with LWD and other data (see “Logging while drilling,” “Physical properties,” “Structural geology,” and “Geochemistry”), with depth shifts of ~50–70 m compared to LWD data, 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 that makes it difficult to accurately reconstruct stratigraphic relationships. As sand was recovered in cuttings and drilling fluid as mostly unconsolidated material, the >63 µm sand 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 Hole C0002F, defining units and subunits by the first occurrence of a change in cuttings lithology (e.g., the first appearance of sandstone) is the most reasonable approach. Because of smearing effects created by the first cut by the drill bit and the last cut by the underreamer, as well as by general circulation of cuttings fragments, the base of a unit or subunit can be defined only in an imprecise way by the last common occurrence or the last occurrence of a lithology such as sandstone. In effect, the upper boundaries are clearly defined, whereas the lower boundaries are more arbitrary. Because of this complexity and for consistency, lithologic upper boundaries are defined by the first occurrence of a lithology (i.e., sandstone) and lower boundaries are defined by the first appearance of the lithology of the immediately subjacent unit. This approach only allows discrimination of units that have contrasting lithology. Mineralogical and geochemical analysesX-ray diffraction mineralogyBulk powder XRD results show the relative abundance of total clay minerals, quartz, feldspar, and calcite. As a measure of how accurate the XRD estimates are relative to absolute percentages, regression analysis of percent calcite from XRD versus percent calcium carbonate from coulometric analysis is shown in Figure F27 (see also “Organic geochemistry”). The linear regression coefficient (R2) shows a very good correlation of 0.97. The comparison also shows a slight shift in the coulometric data above ~10 wt%, which is to be expected if the concentration of CaCO3 is expressed as a percentage of the total solid mass (weight percent) and calcite measured by XRD is normalized to 100 wt%. Figure F28 and Table T9 show XRD data of cuttings from the 1–4 mm and >4 mm size fractions. No significant differences are apparent between cuttings size fractions; consequently, we continued to only analyze cuttings from the 1–4 mm size fraction (also in line with standard oil industry cuttings routines). Regularly spaced >4 mm samples were analyzed for quality control. Because of the mixing of cement and formation in the upper part of the hole (see “Physical properties”), XRD data were routinely measured starting at 920.5 mbsf (Sample 338-C0002F-20-SMW). The uppermost few measurements still show contamination with cement, especially in the >4 mm size fraction. Because of drilling with the underreamer, we observe a gradual increase between 920 and 1025.5 mbsf (Samples 20-SMW through 45-SMW) in total clay from ~32 to 58 wt% and in feldspar from ~12 to 28 wt% as well as a large decrease in calcite from ~28 to 5 wt%. Because of this gradual decrease in calcite, together with the first occurrence of sandstone, the lithologic Unit III/IV boundary is defined at 1025.5 mbsf (Sample 45-SMW). This boundary is defined by LWD data at ~918.5 mbsf (see “Logging while drilling”), whereas Expedition 315 observed an abrupt reduction in calcite content at the discordance at 922 mbsf (Expedition 315 Scientists, 2009b). This shift was explained during Expedition 315 as the abrupt change of the depositional site from below (lithologic Unit IV) to above the carbonate compensation depth (CCD) (lithologic Unit III). Similar but more gradual shifts in calcite content were also recorded from Ocean Drilling Program (ODP) Sites 1175 and 1176 (Shipboard Scientific Party, 2001a, 2001b; Underwood et al., 2003). Lithologic Unit IV, which is divided into five subunits (IVA–IVE), shows five cycles of increasing then decreasing total clay content that correspond reasonably well with the subunit boundaries (Fig. F28). The amount of quartz (weight percent) remains relatively constant with a slight but not significant increase and then decrease within the subunits. Feldspar shows a broad distribution throughout lithologic Unit IV with no clear trend but some subtle changes at the subunit boundaries. Calcite content remains low with a more substantial decrease to ~1–2 wt% in lithologic Subunits IVB and IVC followed by an increase in lithologic Subunit IVE. More detailed observations can be summarized as follows. In lithologic Subunit IVA, total clay mineral content shows little variation with an average of ~58 wt%, quartz averages ~20 wt%, feldspar slightly increases from ~20 to 22 wt%, and calcite is low at ~3 wt%. At the lithologic Subunit IVA/IVB boundary (1140.5 mbsf; Sample 338-C0002F-71-SMW), total clay, quartz, feldspar, and calcite show more scatter in the data but no significant downhole changes or trends. In lithologic Subunit IVB, total clay and quartz show similar values to those in lithologic Subunit IVA, but feldspar shows a slight increase and greater scatter. Calcite values remain similarly low (average = 3 wt%). In lithologic Subunit IVC at 1270 mbsf (Sample 338-C0002F-SMW-100), total clay content increases from an average of 44 wt% to 58 wt%, quartz increases slightly, and feldspar shows considerable scatter in the data. Calcite content drops to an average ~1 wt%. In lithologic Subunit IVD from 1420.5 to 1600.5 mbsf (Samples 338-C0002F-134-SMW to 182-SMW), total clay increases and then decreases slightly, quartz content increases slightly, feldspar content decreases, and calcite values remain low. In lithologic Subunit IVE between 1600.5 and 1740.5 mbsf (Samples 338-C0002F-182-SMW to 215-SMW), when compared with lithologic Subunit IVD data, total clay content decreases then increases, quartz content increases then decreases, feldspar values remain essentially constant, and calcite increases slightly. The lithologic Unit IV/V boundary at 1740.5 mbsf (Sample 215-SMW) is associated with an increase in total clay content. Within lithologic Unit V there is a further increase in clay mineral content at 1930.5 mbsf (Sample 269-SMW). Quartz content slightly increases at the Unit IV/V boundary and then decreases at 1860.5 mbsf (Sample 253-SMW). Feldspar decreases throughout lithologic Unit V to an average of ~12 wt%. Calcite decreases from 5 to 1 wt% until 1855.5 mbsf (Sample 254-SMW), where it increases again to an average of ~8 wt% before decreasing at 1930.5 mbsf (Sample 269-SMW) to an average of ~2 wt%. All mineral data taken from cuttings in Hole C0002F correlate well with the core data from Site C0002B (450–1050 mbsf) (Fig. F29). In comparison with core data, the XRD data from cuttings are relatively homogeneous because of the preferential preservation of the fine-grained (more indurated) sediment in the silty claystone (1–4 mm size fraction) with respect to coarse-grained (less indurated) sandy sediment. Among the major minerals, calcite (XRD) shows the greatest amount of scatter. This is similar to observations made from Site C0002B (Expedition 315 Scientists, 2009b), where calcite abundance ranges from 0.63% (trace) to 27.16% with an average of 14.21%. X-ray fluorescenceIn order to characterize compositional trends with depth and/or lithologic characteristics of the sediments from Hole C0002F, X-ray fluorescence (XRF) analysis was undertaken for ~150 samples (Fig. F30; Table T10). Major and minor element contents (SiO2, Al2O3, CaO, K2O, Na2O, Fe2O3, MgO, TiO2, P2O5, and MnO) were analyzed and complemented by loss on ignition (LOI) measurements. To compare the composition of cuttings sizes, initially both 1–4 mm and >4 mm cuttings size fractions were analyzed. A comparison shows no significant differences for cuttings size fractions; therefore, further analysis only involved the 1–4 mm cuttings size fraction. The compositional spikes observed in the upper interval in Hole C0002F for the >4 mm cuttings size fraction are mainly due to the mixing of cement and formation (Fig. F30). LOI within the zone of 100% cement ranges up to 25.4 wt%, and such samples are not plotted or used in assessing averages. Elemental compositions are described based on the results of the 1–4 mm cuttings size fraction. LOI averages 9.0 wt% with a maximum of 13 wt% at 920.5 mbsf (Sample 338-C0002F-20-SMW) and a minimum of 6.7 wt% at 1670.5 mbsf (Sample 201-SMW). The abundance of SiO2 is high throughout Hole C0002F with an average of 64.1 wt% and varying from a minimum of 58.5 wt% at 920.5 mbsf (Sample 338-C0002F-20-SMW) to a maximum of 67.84 wt% at 1330.5 mbsf (Sample 112-SMW). SiO2 shows a reasonably good correlation with the other element oxides, such as Al2O3, Na2O, K2O, and CaO (Fig. F31). Al2O3 averages 15.9 wt% with a minimum of 13.7 wt% at 925.5 mbsf (Sample 338-C0002F-21-SMW) and a maximum of 17.11 wt% at 1025.5 mbsf (Sample 45-SMW). CaO averages 4.30 wt% with a minimum of 2.10 wt% at 1870.5 mbsf (Sample 255-SMW) and a maximum of 12.0 wt% at 920.5 mbsf (Sample 20-SMW). K2O averages 3.3 wt% with a minimum of 2.60 wt% at 920.5 mbsf (Sample 20-SMW) and a maximum of 3.6 wt% at 1990.5 mbsf (Sample 286-SMW). Na2O averages 2.5 wt% with a minimum of 2.1% at 1970.5 mbsf (Sample 282-SMW) and a maximum of 2.8% at 1010.5 mbsf (Sample 42-SMW). In common with the other element oxides, Fe2O3 shows no clear trend with depth and averages 5.3 wt% with a minimum of 4.5 wt% at 1430.5 mbsf (Sample 338-C0002F-136-SMW) and a maximum of 5.9 wt% at 1110.5 mbsf (Sample 64-SMW). MgO averages 2.2 wt% with a minimum of 1.85% at 1670.5 mbsf (Sample 201-SMW) and a maximum of 2.60% at 1010.5 mbsf (Sample 42-SMW). TiO2 averages 0.64 wt% with a minimum of 0.58 wt% at 1430.5 mbsf (Sample 136-SMW) and a maximum of 0.71 wt% at 1000.5 mbsf (Sample 41-SMW). MnO averages 0.065 wt% with a minimum of 0.05 wt% at 1890.5 mbsf (Sample 338-C0002F-260-SMW) and a maximum of 0.09 wt% at 1040.5 mbsf (Sample 49-SMW). P2O5 averages 0.09 wt% with a minimum of 0.06 wt% at 1910.5 mbsf (Sample 265-SMW) and a maximum of 0.13 wt% at 1040.5 mbsf (Sample 48-SMW). Figure F31 shows cross-plots for various element oxides. These graphs contain two distinct, nonoverlapping populations of data (labeled “Population 1” and “Population 2”). Population 1 consists of the data from 920.5 to 990.5 mbsf (Samples 338-C0002F-20-SMW through 36-SMW), and Population 2 contains all data from 995.5 to 1990.5 mbsf (Samples 37-SMW through 286-SMW). It is likely that Population 1 represents contamination from the cement, whereas Population 2 reflects essentially formation geochemical data. SiO2 shows a positive correlation with Al2O3 (Fig. F31A). CaO shows a negative correlation with both SiO2 (Fig. F31B) and Al2O3 (Fig. F31C). Al2O3 shows a negative correlation with K2O (Fig. F31D). LOI shows a positive correlation with CaO (Fig. F31E). Interpretation of drilled stratigraphyLithologic Unit III, consisting of silty claystone with trace amounts of sandy material, is interpreted as the fill of the lower part of the Kumano forearc basin and potentially prism slope basins (Expedition 315 Scientists, 2009b). 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). Lithologic Unit IV represents the uppermost part of the older accretionary prism sediment with silty claystone as the major lithology. Sandstone tends to consist of mainly quartzo-feldspathic material, including metamorphic rock fragments, common heavy-mineral assemblages, relatively rare ferromagnesian minerals, variable but generally small amounts of organic/wood material, and traces of volcanic glass. This assemblage is consistent with proximity to a volcanic source. Expedition 315 interpreted lithologic Unit III as forearc or supra-accretionary prism slope deposits that accumulated above the CCD, both prior to and during the early stages of formation of the Kumano Basin (Expedition 315 Scientists, 2009b). 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. Expedition 315 proposed that the base of lithologic Unit III is a depositional contact between accreted trench-wedge sediment and the initial deposits of hemipelagic silty claystone on the lowermost trench slope (Expedition 315 Scientists, 2009b). 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 (Expedition 315 Scientists, 2009b; their figure F4) as a manifestation of uplift along a system of out-of-sequence (splay) faults that occurred at ~5 Ma. Whether the uplift triggered erosion of accreted strata or favored slow sediment accumulation 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 sediment 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 inception of lithologic Unit II deposits (Expedition 315 Scientists, 2009b). During Expedition 315, the depositional environment of lithologic 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, 2009b). Seismic reflection data indicate that the contact between lithologic Units III and IV is a boundary between the forearc basin and the older accretionary prism, which means that the most likely depositional environment for lithologic Unit IV is older accretionary prism slope basin or trench wedge. Low concentrations of calcareous nannofossils suggest deposition below the CCD in a slope basin 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). Lithologic Unit IV consists of the most sandstone-rich deposits recovered in Hole C0002F. 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. In lithologic Unit IV, the presence of the high-temperature metamorphic mineral corundum at 1125.5 mbsf (Sample 338-C0002F-66-SMW) (Fig. F25A, F25B), a characteristic mineral of contact metamorphism of limestones and metamorphosed shales (e.g., schists), likely came from the Jurassic low-pressure/high-temperature Ryoke Metamorphic Belt. If correct, then its presence may indicate a sequential unroofing history from the Shimanto Belt to the older and more deeply buried Ryoke Belt. The lithologic Unit IV/V boundary at 1740.5 mbsf (Sample 338-C0002F-215-SMW) is identified as an important candidate thrust zone (see “Logging while drilling”). XRD and XRF analyses show a significant shift in mineralogy and element oxides at this interface. For XRF analyses (Fig. F30), the shift to increased values for LOI, CaO, MgO, and P2O5, with an opposite shift for SiO2, Al2O3, K2O, Na2O, Fe2O2, MgO, and TiO2, can be explained by ion-rich fluid migrating along the thrust zone to precipitate Ca-Mg clay minerals. Lithologic Unit V consists essentially of silty claystone as the finest grained deposits within any unit in Hole C0002F, also associated with the highest gamma radiation values (see “Logging while drilling”). Its thickness, several hundred meters, suggests that it is a candidate correlative unit to the hemipelagic lithologic Unit III drilled at subduction inputs Sites C0011 and C0012 (Expedition 322 Scientists, 2010a, 2010b), albeit possibly internally thrust duplicated and folded. Hole C0002HTwo cores were recovered in Hole C0002H (Table T11). Core recovery was limited: ~18.4% in Core 338-C0002H-1R and 22.7% in Core 2R (see “Background and objectives”). Despite the difficulty with recovery and the consequent expectation of the preferential loss of unconsolidated sandy materials, ~27% of the recovered interval is weakly consolidated sandstone. The small amount of core recovered precludes identification of stratigraphically meaningful units and subunits, so we focus here on a detailed description of the two cores. The depth interval cored is situated in Subunit IVA (Hole C0002F; 1025.5–1140.5 mbsf) and suggests that these materials were obtained close to the lithologic Subunit IVA/IVB boundary as shown on Figure F18. Further comparison to Hole C0002F is discussed below. Lithologic variationThe dominant lithology in both cores is dark greenish gray silty claystone (Figs. F32, F33). Minor lithologies include sandstone, sandy siltstone, and calcareous claystone. Silty claystones are consolidated to the point that they cannot be fully disaggregated by standard smear slide methodology (Fig. F34A). However, coherent fragments are sufficiently small that they can be usefully examined in transmitted light. All the lithologies are dominated by a siliciclastic grain assemblage of clay, quartz, and feldspar (see Hole C0002H smear slides in “Core descriptions”). Lithic fragments are comparatively minor and consist mostly of sedimentary (fine-grained siliciclastic lithics and chert) and low-rank metamorphic clasts such as slate and phyllite. Minor mineral grains include micas (mostly biotite and chlorite) and a diverse assemblage of dense minerals. Examples of carbonate-bearing silty claystone are observed in intervals 338-C0002H-1R-1, 42–46 cm, 51–56 cm, and 96–103 cm. Carbonate is primarily present in the form of nannofossils (Fig. F34B) and as silt-size anhedral calcite and ranges from a trace in the dominant silty claystone to 30% in the more calcareous lithology, based on smear slide observations (see Hole C0002H smear slides in “Core descriptions”), XRD measurements (Table T12), and carbonate analyses (Table T13). A localized detrital component, concentrated fragments of terrestrial organic matter (Fig. F34C), occurs as sequences of laminae of 2–3 mm thickness in intervals 1R-1, 48–50 cm; 2R-3, 70–75 cm; and 2R-3, 103–107 cm (Figs. F32, F33). Biological featuresThe above-noted nannofossils are dominantly moderately well preserved coccoliths and minor discoasters. Only a trace of highly fragmented siliceous bioclasts, including sponge spicules and radiolarians, was observed. Skeletal fragments of any type are rare. Agglutinated tubes of a possible large foraminifer are observed (~0.5 cm diameter) scattered throughout both cores. Generalized bioturbation is observed throughout both cores, but particular ichnotaxa were not identified. Small (millimeter-scale) pyritized burrows are especially visible in the X-ray computed tomography (CT) images. Zones of intense burrowing are particularly well developed beneath the calcareous layers, which are themselves highly bioturbated. The X-ray CT image (Fig. F35) reveals that some burrows within noncalcareous silty claystone are filled with calcareous silty claystone from the overlying calcareous layer and also that some burrows appear to cross the lithologic boundary. Authigenic componentsFew authigenic components can be recognized in silty claystone using light microscopy. Pyrite framboids (Fig. F34C) are widely distributed through both cores. Possible microdolomite (Fig. F34D) of very uniform crystal size (1–3 µm) observed in the calcareous silty claystone at Section 338-C0002H-1R-1, 105 cm, possibly contributes to the high X-ray CT density that is observed for that lithology (Figs. F32, F35). The only macroscopically apparent authigenic feature is a drilling-deformed fragment of calcite-cemented sandstone surrounded by unconsolidated sand in interval 2R-1, 40–44 cm, that also displays high density on the X-ray CT image. Comparison to Hole C0002H and other sites on the Nankai marginLithologies observed in Cores 338-C0002H-1R and 2R are consistent with the range of lithologies observed in Hole C0002F, and specifically, the sand-rich lithologic Subunit IVB (Fig. F18). The sandstone proportion recovered in Hole C0002H is most likely less than the actual stratigraphic percentage of sand as a consequence of sand loss during core recovery; there also may be some influence from underreamer mixing. XRD and XRF compositions (Figs. F36, F37; Tables T12, T14) are generally similar to those observed in cuttings in Hole C0002F but show far more scatter, as expected for discrete samples versus cuttings, because of the homogenization from mixing of different lithologies in the cuttings. On a broader scale, lithologies recovered in Cores 338-C0002H-1R and 2R are similar to lithologies reported at IODP Site C0001 in lithologic Subunit IC (basal slope apron) and Unit II (accretionary prism) and in Hole C0002B in lithologic Unit IV (accretionary prism) (Expedition 315 Scientists, 2009a, 2009b) with the exception that the sand percentage recovered is somewhat higher in Hole C0002H, more similar to the sand-rich character observed in Hole C0002F lithologic Subunit IVB. Lithologies, major and minor grain components, and biologic components are all consistent with the features described more widely on the Nankai margin (e.g., Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009) with the notable exception that volcanogenic material (volcanic lithic fragments, pumice, and volcanic glass) is very minor to absent in Hole C0002H. The loss of sand during drilling and coring and the general high level of drilling-induced core disturbance and structural deformation (see “Structural geology”) prevent clear recognition of the characteristic sedimentary depositional successions in these cores. A few fining-upward sequences, capped by calcareous silty claystone (intervals 338-C0002H-1R-1, 52–54 cm; 1R-2, 3–11 cm; 2R-1, 16 cm; 2R-3, 42 cm; 2R-3, 68 cm; 2R-3, 69–72 cm; 2R-3, 85–89 cm; and 2R-3, 105–114 cm) suggest the presence of turbidites a few centimeters or tens of centimeters thick, with most now missing their sand (presumably lost during core recovery), that grade from fine sand or coarse silt at the base to more nannofossil rich silty claystone that has been greatly obscured by bioturbation. The ratio of siliciclastic debris to pelagic components suggests a relatively higher rate of sedimentation compared to the condensed mudrock succession in lithologic Unit III (Hole C0002B, Expedition 315 Scientists, 2009b). The abundant bioturbation in the silty claystones suggests deposition under conditions of normal seafloor oxygenation. Hole C0002JSeven cores were recovered in Hole C0002J (Table T15) with an average recovery of ~60% (see “Background and objectives”). The depth interval cored (902–940 mbsf), based on comparisons to logs from Hole C0002A (Expedition 314 Scientists, 2009), logs and cuttings from Hole C0002F (Fig. F18), and cores from Hole C0002B (Expedition 315 Scientists, 2009b), suggests that these materials were obtained close to the lithologic Unit III/IV boundary. Given the relatively short interval cored and the limited recovery, we focus on description of Hole C0002J cores and possible stratigraphic correlations of the cored interval and sediments observed in Holes C0002B and C0002F. Specifically, we focus on characterization of a possible unit boundary (lithologic Unit III/IV) in Section 338-C0002J-5R-8. Conclusions on the nature and exact position of this boundary will be further refined though postexpedition research. Lithologic variationThe dominant lithology in Hole C0002J is dark olive-gray silty claystone (Fig. F38) (see Hole C0002J smear slides in “Core descriptions”). Minor lithologies include sandstone, sandy siltstone, silty claystone, calcareous claystone, and fine ash. XRD and XRF data show that bulk mineralogical and bulk elemental compositions are broadly similar to those observed at this depth interval in Hole C0002B (Figs. F39, F40), with a relatively sharp drop in carbonate content at the possible lithologic Unit III/IV boundary (see further discussion below). All the lithologies are dominated by a siliciclastic grain assemblage of clay, quartz, and feldspar with variable amounts of pelagic carbonate (Tables T13, T16, T17). Lithic fragments consist mostly of sedimentary (fine-grained siliciclastic lithics and chert) and low-rank metamorphic clasts such as slate and phyllite (Fig. F41). Minor mineral grains include micas (mostly biotite and chlorite) and a diverse assemblage of dense minerals. Volcanic glass is widely distributed in the silty claystone and also in the coarser lithologies. Vitric material is mostly silt-size clear glass, but subordinate amounts of brown glass, microlitic volcanic rock fragments, and pumice (Fig. F42) are present locally. Carbonate is primarily present in the form of nannofossils and also as silt-size anhedral calcite and ranges from trace in the dominant silty claystone to 20% in the more calcareous lithology, based on smear slide observations (see Hole C0002J smear slides in “Core descriptions”), XRD measurements (Table T16), and carbonate analyses (Table T13). Minor amounts of terrestrial organic matter (red-brown color) are observed in the coarser lithologies. Biological featuresThe above-noted nannofossils are dominantly moderately well preserved coccoliths and minor discoasters. Samples generally contain trace to minor amounts of highly fragmented siliceous bioclasts, including sponge spicules and radiolarians. Skeletal fragments of any type are rare. Agglutinated tubes of a possible large foraminifer are observed (~0.5 cm in diameter) scattered through the core. Generalized bioturbation is pervasive and most readily appreciated in X-ray CT images. Small (millimeter-scale) pyritized burrows similar to Trichichnus (McBride and Picard, 1991) are the most common type of discrete burrow, but Chondrites, Zoophycos, and other discrete burrows are well preserved in local zones, most especially in Cores 338-C0002J-1R through 5R (Fig. F43). Numerous occurrences of possible syndepositional erosion are observed in Cores 338-C002J-4R through 7R (Fig. F44), including angular mud clasts and scoured bedding surfaces that display a range of inclinations. Authigenic componentsFew authigenic components can be recognized in silty claystone using light microscopy. Pyrite framboids are widely distributed through all the cores and are most notably developed within and around burrows as noted above. Glauconite is also notable in Cores 338-C002J-1R to 5R and occurs in a variety of forms (Fig. F45). Slightly wavy greenish bands 1–3 cm thick are most likely slightly glauconized silty claystone, although the specific form of the glauconite is not discernible in smear slides. Glauconite also occurs as discrete grains of silt to granule size (Fig. F45C, F45E) that appear in smear slides as grass-green claystone and silty claystone (Fig. F45D). The interval 338-C0002J-6R-1, 15–23 cm, has a zone of calcareous sandy mudstone composed of very uniform anhedral calcite microcrystals that form a matrix around sand grains (Fig. F46). This material is similar to possible authigenic calcite (microbial precipitate?) encountered in Sample 315-C0002B-59R-1, 45–52 cm (see “Core descriptions”), within the upper part of lithologic Unit IV (Expedition 315 Scientists, 2009b). This lithology is the source of the rare carbonate-rich material observed locally within lithologic Unit IV. Possible unit boundaryA possible unit boundary (lithologic Unit III/IV; Kumano Basin/prism) is identified within a zone ~18 cm thick, beginning at ~926.66 mbsf in Section 338-C0002J-5R-8 (Fig. F47). Interpretation as a unit boundary is based on lithologic evidence, compared for reference to core observations made in Hole C0002B (Expedition 315 Scientists, 2009b), together with the proximity to the boundary depth observed by sampling and logging in Holes C0002A, C0002B, and C0002F. Lithologic evidence for the boundary includes the following:
Ash occurs both above and below the boundary. Although ash is a persistent component of silty claystones above the boundary, the ash occurrence in both silty claystone and sandstone below the boundary is more variable, ranging from abundant in zones adjacent to ash beds to near absent in beds farther from the ash. In Section 338-C0002J-5R-8, immediately above and within the boundary zone, evidence for erosion, as described above and depicted in Figure F44, becomes pronounced (Fig. F47). Bulk elemental compositional variation across the possible unit boundary in Section 338-C0002J-5R-8 was examined using XRF core scanning (Fig. F47; Table T18). The lithologic Unit III/IV boundary may not be a single sharp contact (as in Hole C0002B). Instead, it may occur as a zone of heterogeneous lithology, containing alternations of materials from lithologic Units III and IV. This variety is also reflected in the XRF core scanning results. The peaks in Fe2O3 correspond to levels rich in glauconite. Al2O3 yields a noisy signal, but greater Al2O3 clearly corresponds to carbonate-poor claystone. Biostratigraphic data for Holes C0002B, C0002F, and C0002J results indicate the presence of middle Pliocene sediment at 925.48 mbsf (see “Biostratigraphy”; Tables T19, T20, T21), indicating that indeed the transition to prism sediment of likely late Miocene age occurs below this depth. Sediment below our proposed boundary at 926.7 mbsf have not, to date, yielded datable nannofossil material. It remains possible that coring in Hole C0002J did not penetrate the lithologic Unit III/IV boundary, as glauconitic materials (possibly affiliated with lithologic Unit III) are observed in the deepest section cored (Section 7). Carbonate content, however, provides stronger evidence that lithologies from Unit IV have been encountered. Although minor amounts of carbonate-bearing silty claystone have been observed in the upper part of lithologic Unit IV (mentioned in “Authigenic components”), claystones as calcite poor as the interval 338-C0002J-5R-8, 102–106 cm, have not been previously reported in lithologic Unit III (this chapter and Expedition 315 Scientists, 2009b). Comparison to other data on the basin/prism boundaryPlacement of the lithologic Unit III/IV boundary in Section 338-C0002J-5R-8 (Fig. F47) can be examined in the context of previous observations of the contact between basinal sediment and the prism (Table T22). A transition from calcareous mudstone in the basinal sediment to carbonate-poor mudstone in the prism is a theme that recurs across all the sampled holes at Site C0002 (Holes C0002B, C0002F, C002H, and C0002J) as well as at Site C0001. The depth of the tentative boundary placement in Hole C0002J is consistent with lithologic differences observed in silty claystones across this boundary in both Holes C0001H and C0002B, although the amount of sand observed below the boundary in Hole C0002J is greater. An increase in the amount of sand below the boundary is, however, consistent with observations made in logs and cuttings in Hole C0002F. The depth of the boundary tentatively identified in Hole C0002J, however, matches the log-identified boundary in Hole C0002F more closely than the boundary identified based on lithology. Biostratigraphic and paleomagnetic evidence indicate that the boundary as observed in Holes C0001H and C0002B is a significant unconformity. These observations are consistent with the evidence for syndepositional erosion documented here and are also consistent with the possibility that the boundary is variable in terms of the character of the lithologic transitions and the topography at the contact. Holes C0002K and C0002LThe coring interval in Holes C0002K and C0002L was chosen to provide data within a gap (200–500 mbsf) that was not cored during Expedition 315. Based on comparisons to logs for Hole C0002A (Expedition 314 Scientists, 2009) and cores from Hole C0002B (Expedition 315 Scientists, 2009b), materials in this interval are within lithologic Unit II. A total of 35 cores comprising 265 sections were recovered in Holes C0002K and C0002L (Table T23) with an average recovery of ~69% and 79%, respectively (see “Background and objectives”). Lithologic variationThe dominant lithology in Holes C0002K and C0002L is dark olive-gray silty claystone (Fig. F48) (see Hole C0002K and C0002L smear slides in “Core descriptions”). Minor lithologies include sandstone, sandy siltstone, silty claystone, calcareous claystone, and fine ash. Most samples are dominated by a siliciclastic grain assemblage of clay, quartz, and feldspar with variable amounts of pelagic carbonate (Figs. F49, F50; Tables T24, T13, T25) and a minor but persistent component of volcanic glass. Total carbonate content ranges from <1% to ~15% in the dominant silty claystone and up to 30% in the more calcareous silty claystone in the pelagic-influenced upper parts of the turbidite cycles, based on smear slide observations (see Hole C0002K and C0002L smear slides in “Core descriptions”), XRD measurements (Table T24), and carbonate analyses (Table T13). The feldspar is dominantly plagioclase; much of it is untwinned and highly vacuolized. Lithic fragments consist mostly of fine-grained siliciclastic lithics and chert and low-rank metamorphic clasts such as slate and phyllite (Fig. F51). Minor mineral grains include micas (mostly biotite and chlorite) and a diverse assemblage of dense minerals. Volcanic glass is widely distributed in silty claystones and also in coarser lithologies. Vitric material is mostly silt-size clear glass. Carbonate is primarily present in the form of nannofossils and also as foraminifers and silt-size anhedral calcite. Minor amounts of terrestrial organic matter are present. The typical occurrence of sand in these cores takes the form of thin turbidite cycles that vary in sand thickness (Fig. F52; Table T26). Thicker turbidites range from decimeter to meters thick cycles with sand sitting above a scoured base; fining upward into sandy silt, clayey silt, and silty claystone; and capped by a somewhat calcareous silty claystone rich in pelagic debris (coccoliths) (Fig. F52A). Thinner turbidites begin with centimeter-scale silty sand or clayey silt (Fig. F52B), and the smallest ones are represented only by slightly coarser silty claystones at subtly scoured contacts above calcareous silty claystone (Fig. F52C). Across the depth interval sampled, turbidite sand has an uneven distribution (Fig. F53), being more abundant in the zones above 300 mbsf and below 450 mbsf. The thickest sand observed is ~1 m thick. Biological featuresThe above-noted nannofossils are dominantly moderately well preserved coccoliths and minor discoasters. Samples generally contain a trace to minor amounts of highly fragmented siliceous bioclasts, including sponge spicules and radiolarians. A few core sections that are poor in sand (e.g., Cores 338-C0002K-1H and 2H and Sections 338-C0002L-4X-1 through 4X-6) contain biosiliceous components at amounts of a few percent. Generalized bioturbation and discrete burrows are not generally evident, either in core or CT images. Authigenic componentsPyrite framboids are the only commonly observed authigenic component. InterpretationExpedition 315 Scientists (2009b), working from cores with poor recovery of the sand, interpreted lithologic Unit II as the lower forearc basin succession, dominated by the hemipelagic mud of distal turbidites. The somewhat better core recovery achieved in Holes C0002K and C0002L allows us to confirm this interpretation for the upper part of lithologic Unit II. Patterns of sand occurrence are suggestive of the presence of a coarsening-upward package of generally thin turbidites from ~460 mbsf to the top of lithologic Unit II, possibly underlain by a second similar cycle that begins at the top of Core 338-C0002L-22X at ~480 mbsf. Poor core recovery in Hole C0002B precludes immediate assessment of this possibility of large-scale turbidite packages within lithologic Unit II; additional work with core-log integration in postexpedition studies may further elucidate the depth trends of sand in lithologic Unit II. |
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