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doi:10.2204/iodp.proc.338.102.2014

Lithology

At Sites C0002, C0021, and C0022, cuttings description (Hole C0002F), visual core description (Holes C0002H, C0002J, C0002K, C0002L, C0021B, and C0022B), and LWD data (Holes C0002F, C0018, C0021A, and C0022A), including gamma ray, resistivity, and sonic data, were available to identify lithologic boundaries and units. Methods applied to core description during Expedition 338 draw upon the protocols of IODP Expedition 315 (Expedition 315 Scientists, 2009a), whereas methods applied to cuttings description rely heavily upon procedures established during IODP Expedition 319 (Expedition 319 Scientists, 2010b), in particular the Cuttings Cookbook.

Cuttings samples in Hole C0002F were described based on the examination of 70 cm³ aliquots of bulk cuttings. Descriptions included the following:

• Macroscopic observations of percent silty claystone versus percent sandstone,
• Microscopic observations (including smear slides and quartz index measurements [see “Q-index”] from sieved sand fractions), and
• Bulk mineralogical data by XRD and bulk elemental data by XRF.

Core samples in Holes C0002H, C0002J, C0002K, C0002L, C00021B, and C00022B were described based on the following:

• Macroscopic observations following standard IODP visual core description protocols and observation of X-ray CT images,
• Microscopic observations (including smear slides and thin sections), and
• Bulk mineralogical data by XRD, bulk elemental data by XRF, and semiquantitative elemental data by XRF core scanning (Sakamoto et al., 2006).

Depths reported for cores and discrete samples are core depth below seafloor, Method A (CSF-A).

Macroscopic observations of cuttings

Cuttings typically occur as small fragments of rocks, in general 0.25–8 mm in size, often produced as reaggregates of various lithologies fragmented during drilling. Cuttings were taken for the first time in IODP operations during Expedition 319 (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). Sampling and analysis of cuttings follow the Cuttings Cookbook developed during Expedition 319, with some additions and modifications. Cuttings were taken every 5 m from 875.5 to 2005.5 mbsf and separated by laboratory technicians into rock-chip fractions of different sizes (0.25–1 mm, 1–4 mm, and >4 mm). However, at shallow depths, solid fragments from the formation can be suspended in drilling mud and mixed with trace amounts of clay-bearing drilling additives (e.g., bentonite). Rigorous separation of drilling-related mud from formation cuttings is not always possible, especially in the case of very soft cuttings. This hampers quantification of the true clay content. The separation procedure of cuttings from drilling mud, and the division into different sizes, is explained in the Cuttings Cookbook.

Based on general visual observations of the cleaned bulk cuttings material, we estimated the relative amount of silty claystone and sandstone, induration state, shape, occurrence of wood, and amount of artificial contamination. All macroscopic observations were recorded on visual cuttings description forms and summarized in CDS_MACRO_SED.PDF in CUTTINGS in VCD_SCAN in “Supplementary material.”

Macroscopic observations of core

We followed conventional Ocean Drilling Program (ODP) and IODP procedures for recording sedimentological information on Visual Core Description (VCD) forms on a section-by-section basis (Mazzullo and Graham, 1988). Core descriptions were transferred to section-scale templates using the J-CORES database and then converted to core-scale depictions using Strater (Golden Software). Texture (defined by the relative proportions of sand, silt, and clay) follows the classification of Shepard (1954). The classification scheme for siliciclastic lithologies follows Mazzullo et al. (1988).

To emphasize the differences in composition of volcanic sandstones in cuttings and core, we modified the classification scheme of Fisher and Schmincke (1984). In general, coarser-grained sedimentary rocks (63 µm–2 mm average grain size) are named “sandstone,” where volcaniclastic components are <25% of the total clasts. Volcaniclastic grains can be (1) reworked and commonly altered heterogeneous fragments of preexisting volcanic rock, tuff, or tephra or (2) fresh, or less altered, compositionally homogeneous pyroclasts. Pyroclasts are produced by many types of processes associated with volcanic eruptions without reference to the eruption causes or particle origin. Pyroclasts can include crystals, glass shards, and rock fragments. If the sedimentary rock contains >25% but <75% volcaniclasts, it is designated a “volcaniclastic sandstone.” As a subset of volcaniclastic sandstone, if >25% but <75% of the volcaniclasts are vitric pyroclasts, then we used the term “tuffaceous sandstone.” If the total clast composition is >75% pyroclasts, then the sediment is classified as “ash” or, if lithified, as “tuff.” Depending on the grain size and degree of compaction, the nomenclature is adjusted accordingly (e.g., tuff versus ash), as shown in Table T6. Because of problems with accuracy, compositions close to the dividing lines of the classification scheme are problematic. In addition, with the exception of fresh glass shards in the population of pyroclasts, it is difficult to use smear slides to discriminate unequivocally between primary eruptive products and crystals or rock fragments created by the erosion of fresh volcanic material.

Where applicable in cores, bioturbation intensity in deposits was estimated using the semiquantitative ichnofabric index as described by Droser and Bottjer (1986, 1991). The indexes refer to the degree of biogenic disruption of primary fabric such as lamination and range from 1 for nonbioturbated sediment to 6 for total homogenization:

• 1 = no bioturbation recorded; all original sedimentary structures preserved.
• 2 = discrete, isolated trace fossils; <10% of original bedding disturbed.
• 3 = ~10%–40% of original bedding disturbed; burrows are generally isolated but locally overlap.
• 4 = last vestiges of bedding discernible, ~40%–60% disturbed; burrows overlap and are not always well defined.
• 5 = bedding is completely disturbed, but burrows are still discrete in places and the fabric is not mixed.
• 6 = bedding is nearly or totally homogenized.

The ichnofabric index in cores was identified with the help of visual comparative charts (Heard and Pickering, 2008). Distinct burrows that could be identified as particular ichnotaxa were also recorded.

The Graphic lithology column on each VCD plots all beds that are ≥2 cm thick to scale. Interlayers <2 cm thick are identified as laminae in the Sedimentary structures column. It is difficult to discriminate between the dominant lithologies of silty claystone and clayey siltstone without quantitative grain size analysis; therefore, we grouped this entire range of textures into the category “silty claystone” on all illustrations. A more detailed description of rock texture was attempted on the smear slide description sheets (see smear slides for each site in “Core descriptions”). We did not use separate patterns for more heavily indurated examples of the same lithologies (e.g., silty clay versus silty claystone) because the dividing line is arbitrary. Figure F7 shows the graphic patterns for all lithologies encountered during Expedition 338. Also shown are symbols for sedimentary structures, soft-sediment deformation structures, severity of core disturbance, and features observed in X-ray CT images in both soft sediment and indurated sedimentary rock.

X-ray computed tomography

X-ray CT imaging provided real-time information for core logging and sampling strategies. A similar methodology to that used during IODP Expedition 316 was followed (Expedition 316 Scientists, 2009a). All core samples during this expedition were routinely scanned with the X-ray CT. X-ray CT scanning was performed immediately after core cutting so that time-sensitive whole-round samples (e.g., those for interstitial water) could be included in this screening process.

The scans were used to provide an assessment of core recovery, determine the appropriateness of whole-round and interstitial water sampling (avoid destructive testing on core samples with critical structural features), identify the location of subtle features that warrant detailed study and special handling during visual core description and sampling, and determine the 3-D geometry, crosscutting and other spatial relations, and orientation of primary and secondary features. See “X-ray computed tomography” for details about the X-ray CT methods.

Microscopic observation of cuttings

Microscopic investigations of the washed >63 µm sand-size fraction of cuttings samples using a binocular microscope allowed us to distinguish different minerals in the sediments; their abundance, roundness, and sorting; and the relative abundances of wood/lignite fragments and fossils. These data are summarized in “Lithology” and Figure F20, both in the “Site C0002” chapter (Strasser et al., 2014b), and in CDS_MICRO_SED.PDF in CUTTINGS in VCD_SCAN in “Supplementary material.” Errors can be large, especially for fine silt and clay-size fractions. Thus, it would be misleading to report values as exact percentages. Instead, the visual estimates are grouped into the following categories:

• D = dominant (>50%).
• A = abundant (>10%–50%).
• C = common (>1%–10%).
• F = few (0.1%–1%).
• R = rare (<0.1%).

Q-index

An additional means of characterizing the sediment is the introduction of a new parameter called the “quartz index” (Q-index). Although the overall sandiness is measured by the percent of silty claystone versus percent of sandstone, the Q-index is a measure of the bulk sand fraction or caliber (i.e., the bulk mean grain size of the sand fraction). For example, it is possible to have a thick stratigraphic section of fine-grained sandstone (high percent of sandstone but a relatively low Q-index) or a relatively thin section of coarse-grained sandstone (low percent of sandstone and relatively high Q-index). Thus, it is important to appreciate that these parameters do not necessarily measure the same lithologic attributes. When comparing the Q-index with the percent of silty claystone versus percent of sandstone, it is apparent that there is a reasonable correspondence (see Figs. F18, F22, both in the “Site C0002” chapter [Strasser et al., 2014b]). For example, lithologic Units III and V at Site C0002 both have higher silty claystone versus sandstone content and a lower Q-index, and lithologic Unit IV has higher sandstone versus silty claystone content and a higher Q-index.

To obtain the Q-index, the sieved and washed >63 µm fraction representing a 5 m cuttings interval was inspected under the binocular microscope. The largest quartz grain in the field of view was selected (ignoring any exceptionally outsized grains) and photographed. The long axis of the grain was measured with the line measuring tool on the Digital Sight microscope camera. In cases of many large grains of similar size, several were measured and the largest one was chosen to represent the Q-index. The exclusion of the larger “rogue grains” does not affect the trends or relative grain sizes in this case because such solitary outsize grains always occur in sand fractions with the largest grain-size populations.

Smear slides

Smear slides are useful for identifying and reporting basic sediment attributes (texture and composition) in both cuttings and cores samples, but the results are semiquantitative at best (Marsaglia et al., 2013). We estimated the abundance of biogenic, volcaniclastic, and siliciclastic constituents using a visual comparison chart (Rothwell, 1989). Cuttings pieces were chosen for smear slide production based on the dominant lithology present in a given interval. If a distinct minor lithology was abundant, an additional smear slide was made for that interval. For cuttings, we estimated the percentage of minerals observed, normalized them to 100%, and reported the results in “Core descriptions.”

For cores, estimates of sand, silt, and clay percentages are entered into the J-CORES database using the Samples application along with abundance intervals for the observed grain types, as given above. Additional observations, including visual estimates for normalized percentages of grain size and mineral abundance, are recorded on the written smear slide forms, which are scanned and provided as supplementary data (see CORES in SCANS in SS_TS in “Supplementary material.”). The sample location for each smear slide was entered into the J-CORES database with a sample code of SS using the Samples application.

The relative abundance of major mineralogy was also validated by XRD (see “X-ray diffraction”), and the absolute weight percent of carbonate was verified by coulometric analysis (see “Geochemistry”).

Smear slides were observed in transmitted light using an Axioskop 40A polarizing microscope (Carl Zeiss) equipped with a Nikon DS-Fi1 digital camera.

Thin sections

Thin sections were prepared for microscopic studies of mineralogy, petrology, paleontology, internal structures, and fabrics of rocks and sediments. A thin section was prepared as a 30 µm (0.03 mm) thick slice of core or cuttings sample. The standard size of billets for thin section preparation was 2 cm × 3 cm × 0.8 cm.

Soft sediments, cuttings, and rocks that were altered, badly weathered, or contained high clay content were dried first in the freeze dryer and then impregnated under vacuum (Epovac) with epoxy (Epofix) prior to mounting. Core or cuttings samples were attached to a glass slide with Petropoxy 154. Before microscopic observation, thin sections were covered by a cover glass using index oil. Thin sections were observed in transmitted light using an Axioskop 40A polarizing microscope (Carl Zeiss) equipped with a Nikon DS-Fi1 digital camera.

X-ray diffraction

The principal goal of XRD analysis of cuttings and cores was to estimate the relative weight percentages of total clay minerals, quartz, feldspar, and calcite from peak areas. For cuttings, XRD analysis was conducted on 10 g samples of the 1–4 mm size fraction every 5 m. This same 10 g sample provided aliquots for analysis of bulk carbonate and XRF elemental chemistry. Some measurements were also made on the >4 mm size fraction for comparison (Samples 338-C0002F-20-SMW through 289-SMW). For cores, material for XRD analysis was obtained from a 10 cm³ sample that was also used for XRF and carbonate analyses. All samples were vacuum-dried, crushed with a ball mill, and mounted as randomly oriented bulk powders. Routine XRD analyses of bulk powders were performed using a PANalytical CubiX PRO (PW3800) diffractometer. XRD instrument settings were as follows:

• Generator = 45 kV.
• Current = 40 mA.
• Tube anode = Cu.
• Wavelengths = 1.54060 Å (Kα₁) and 1.54443 Å (Kα₂).
• Step spacing = 0.005°2θ.
• Scan step time = 0.648 s.
• Divergent slit = automatic.
• Irradiated length = 10 mm.
• Scanning range = 2°–60°2θ.
• Spinning = yes.

In order to maintain consistency with previous NanTroSEIZE results, we used the software MacDiff 4.2.5 for data processing (www.ccp14.ac.uk/​ccp/​ccp14/​ftp-mirror/​krumm/​Software/​macintosh/​macdiff/​MacDiff.html). We adjusted each peak’s upper and lower limits following the guidelines shown in Table T7. Calculations of relative mineral abundance utilized a matrix of normalization factors derived from integrated peak areas and singular value decomposition (SVD). As described by Fisher and Underwood (1995), calibration of SVD factors depends on the analysis of known weight percent mixtures of mineral standards that are appropriate matches for natural sediments. SVD normalization factors were recalculated during Expeditions 315 and 338 after the diffractometer’s high-voltage power supply and X-ray tube were replaced (Ashi et al., 2008). The mixtures were rerun at the beginning of Expedition 338 (Table T8). Bulk powder mixtures for the Nankai Trough are the same as those reported by Underwood et al. (2003): quartz (Saint Peter sandstone), feldspar (plagioclase), calcite (Cyprus chalk), smectite (Ca-montmorillonite), illite (Clay Mineral Society IMt-2, 2M1 polytype), and chlorite (Clay Mineral Society CCa-2). Examples of diffractograms for standard mixtures are shown in Figure F8.

Average errors (SVD-derived estimates versus true weight percent) of the standard mineral mixtures are as follows: total clay minerals = 3.3%, quartz = 2.1%, feldspar = 1.4%, and calcite = 1.9%. Despite its precision with standard mixtures, the SVD method is only semiquantitative, and results for natural specimens should be interpreted with caution. One of the fundamental problems with any bulk powder XRD method is the difference in peak response between poorly crystalline minerals at low diffraction angles (e.g., clay minerals) and highly crystalline minerals at higher diffraction angles (e.g., quartz and feldspar). Clay mineral content is best characterized by measuring the peak area, whereas peak intensity may more accurately quantify quartz, feldspar, and calcite. Analyzing oriented aggregates enhances basal reflections of the clay minerals, but this is time consuming and requires isolation of the clay-size fraction (<2 µm) to be effective. For clay mineral assemblages in bulk powders, the two options are to individually measure one peak for each mineral and add the estimates together (thereby propagating the error) or to measure a single composite peak at 19.4°–20.4°2θ. Other sources of error are contamination of mineral standards by impurities such as quartz (e.g., the illite standard contains ~20% quartz) and differences in crystallinity between standards and natural clay minerals. For trace quantities of a mineral and peaks with low intensity, use of negative SVD normalization factors may result in negative values of absolute weight percent. In such cases, we inserted the numerical value of 0.1% as a proxy for “trace.”

In the final assessment, calculated mineral abundances should be regarded as relative percentages within a four-component system of clay minerals + quartz + feldspar + calcite. How close those estimates are to their absolute percentages within the total solids depends on the abundance of amorphous solids (e.g., biogenic opal and volcanic glass), as well as the total of all other minerals that occur in minor or trace quantities. For most natural samples, the difference between calculated and absolute abundance percentage is probably between 5% and 10%. To compound the error, the XRD data from cuttings show effects of contamination by drilling fluid. The severity of these artifacts is especially obvious in the calculated values of percent calcite. Figures F28 and F29 and Table T9 are all available in the “Site C0002” chapter (Strasser et al., 2014b).

X-ray fluorescence

XRF analyses were obtained in two modes: analysis of whole-rock powder and scanning of the whole-round core surface on some selected intervals.

Whole-rock quantitative XRF spectrometry analysis was performed for major elements on cuttings and cores. For cuttings, the 1–4 mm size fraction (and in some cases the >4 mm size fraction) was used for these measurements as well as for XRD and carbonate analyses. For cores, material for XRF was obtained from a 10 cm³ sample that was also used for XRD and bulk carbon-nitrogen-sulfur (CNS) analyses.

For both cuttings and cores, all samples were vacuum-dried, crushed with a ball mill, and mounted as randomly oriented bulk powders. Major elements were measured using the fused glass bead method and are presented as weight percent oxide proportions (Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, and Fe₂O₃). An aliquot of 0.9 g of ignited sample powder was fused with 4.5 g of SmeltA12 flux for 7 min at 1150°C to create glass beads. Loss on ignition was measured using weight changes on heating at 1000°C for 3 h. Analyses were performed on the wavelength dispersive XRF spectrometer Supermini (Rigaku) equipped with a 200 W Pd anode X-ray tube at 50 kV and 4 mA. Analytical details and measuring conditions for each component are given in Table T9. Rock standards of the National Institute of Advanced Industrial Science and Technology (Geological Survey of Japan) were used as the reference materials for quantitative analysis. Table T10 lists the results for selected standard samples. A calibration curve was created with matrix corrections provided by the operating software, using the average content of each component. Processed data were uploaded into an Excel spreadsheet and are shown in Figure F30 and Table T10, both in the “Site C0002” chapter (Strasser et al., 2014b).

XRF core scanning analysis was performed using the JEOL TATSCAN-F2 energy dispersive spectrometry–based core scanner (Sakamoto et al., 2006). The Rh X-ray source was operated at 30 kV accelerating voltage and a current of 0.170 mA. Data are reported as total counts on the peak and also as semiquantitative oxide weight percent. Semiquantitative analysis was performed using a 200 s accumulation. The following oxides were measured: Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, and Fe₂O₃. This is the same methodology as the one used during Expedition 316 (Expedition 316 Scientists, 2009a). The archive half was scanned because this technique is nondestructive to the sediment. Section 338-C0002J-5R-8 was scanned at a spatial resolution of 0.5 cm, and the scanning line was located along the center axis of the core section.

Identification of lithologic units

In Hole C0002F, we used LWD data (see details in “Logging while drilling”) in conjunction with analyses of cuttings to identify lithologic units and boundaries. We identified compositional and textural attributes of the formation mainly using NGR data, resistivity and sonic logs, and resistivity images (see “Logging while drilling” for details) along with data from cuttings. After evaluating log data quality through the examination of the potential effects of borehole diameter, borehole conditions, and drilling parameters, we defined units using changes in log responses interpreted to reflect differences in rock properties. For this analysis, NGR, sonic logs, and resistivity logs were the main input. Integrated interpretation of all the available logs focused on (1) definition and characterization of units and unit boundaries, (2) identification of composition and physical properties within each unit, and (3) interpretation in terms of geological features (unit boundaries, transitions, sequences, and lithologic composition).

In Holes C0002H, C0002J, C0002K, C0002L, C0021B, and C0022B we interpreted lithologic units within the cores, as with cuttings, using a broad suite of data including logs, visual core descriptions, smear slides, thin sections, XRD, XRF, CNS analysis, and X-ray CT images.

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