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doi:10.2204/iodp.proc.337.102.2013

Lithostratigraphy

Lithostratigraphic observations of cuttings and core samples from Site C0020 during Expedition 337 were performed using multiple approaches based on

• Macroscopic observations,

• Microscopic observations including smear slides or thin sections,

• Mineralogical and elemental analysis using XRD and XRF, and

• X-ray CT image observation.

The obtained lithostratigraphic data were correlated with data from downhole logging.

Cuttings and core description

Macroscopic observations

Cuttings

Cuttings are small fragments of rocks of various lithologies recovered during riser drilling. Cuttings were sampled for the first time in IODP operations during Expedition 319 (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010) and for the second time during Expedition 337. These solid fragments are suspended in riser drilling mud that contains a considerable amount of clay minerals, such as bentonite, and the resulting contamination introduces uncertainty into the quantification of the true clay content. The separation procedure of cuttings from the drilling mud is described in “Introduction.”

During Expedition 337, cuttings were the only solid samples collected from 635 to 1263 m DRF, whereas cuttings from 1263 to 2466 m DRF were collected together with spot cores. Sampling frequency was every 10 m through the drilled interval (184 samples in total). Using freshwater followed by deionized water, washed cutting samples were sieved into three different size fractions (>4 mm, 1–4 mm, and 0.25–1 mm) and then used for sedimentological description. Based on macroscopic observations of the washed samples (i.e., semiconsolidated and/or consolidated sediments), we estimated the relative amount of coarse-grained (e.g., sand and gravel) and fine-grained (e.g., silt and clay) materials, the induration state of the bulk material, the occurrence of artificial mud/grain contamination, and the presence of wood or lignite fragments, as well as the appearance, softness, size, and relative degree of lithification of rock fragments. The legend we used for cuttings description is shown in Figure F4. Figure F5 shows an example of the macroscopic cuttings description. Sand and gravel were combined to one grain fraction in the macroscopic description. Wood/lignite fragments were the dominating fraction in the coal-bearing unit. Wood/lignite fragments were plotted separately because they are not clastic sediments with a defined grain size.

All macroscopic observations were recorded on visual cuttings description forms and summarized in C0020_T1.XLSX in LITH in “Supplementary material.” In addition, Site C0020 macroscopic visual cuttings descriptions can be found in “Core descriptions.”

Cores

Split cores were described by the shipboard sedimentologists. Visual core description was carried out on the archive half of each core using traditional Ocean Drilling Program (ODP) and IODP procedures (e.g., Mazzullo and Graham, 1988; Mazzullo et al., 1988). The terms “mudstone” and “shale” were used for the description of fine-grained sediments. To keep it simple, we did not differentiate between thin laminated shales and massive mudstones without laminae. Therefore, the terms “shale” and “mudstone” written in the text stand for the same lithotype. Information from the visual core description (VCD) was transferred to the J-CORES database before conversion to core-scale plots using Strater software (Golden Software, Inc., USA). The legend we used for VCDs is shown in Figure F6. An example of the VCD is presented in Figure F7. The content of the core-scale images is shown in Table T2. Scans of handwritten VCD forms entered into the J-CORES database are available in HANDWRITTEN VCD CORE 1-22.PDF and HANDWRITTEN VCD CORE 23-32.PDF in LITH in “Supplementary material.” In addition, Site C0020 visual core descriptions can be found in “Core descriptions.”

A short summary of the key features from each core section was also included in the J-CORES database.

Microscopic observations

Cuttings and cores

Smear slides were made from washed (for semiconsolidated and/or consolidated sediments) cuttings samples in order to identify major lithologic changes under the microscopes. Microscopic investigations of the washed 1–4 mm (coarse sand to granule size) fraction using binocular and/or polarizing microscopes allowed us to distinguish the relative abundances of different minerals, wood/lignite fragments, and fossils in the cuttings. Discrete samples were collected from cores for both smear slide and thin section to identify the main mineral groups and for estimating bulk sediment composition and grain size. These data are summarized in CUTTINGS_STRATER_MOD.XLSX and CORE_STRATER_MOD.XLSX in LITH in “Supplementary material.” In addition, Site C0020 microscopic visual cuttings descriptions can be found in “Core descriptions.”

Smear slides

Examination and description of cuttings and cores during Expedition 337 was undertaken using smear slides. This routine method was used for identifying and reporting basic sediment attributes (i.e., mineralogy, texture, form, and size) and for ascertaining the presence of biological debris in samples of both cuttings and cores. Smear slide samples were taken at regular intervals (i.e., 1 per 10 m in drilling depth for cutting samples and 1 per core section for core samples) or from any lithologically distinct layer. The method included taking a small amount (~0.1 cm) of sample using a toothpick, placing the sample on a clear microscope slide, dispersing with tap water, and then drying on a hot plate. Following drying, optical adhesive was added, covered with a cover glass, and then cured under ultraviolet (UV) radiation before microscope examination.

The estimation of biogenic, volcaniclastic, and siliciclastic abundance in the slide was conducted qualitatively using a visual comparison chart (Rothwell, 1989). In general, the constituents of abundance are reported based on relative percentage:

• D = dominant (>50%).

• A = abundant (>10%–50%).

• C = common (>1%–10%).

• F = few (0.1%–1%).

• R = rare (<0.1%).

The relative abundance of major components was validated by XRD. The absolute weight of carbonate was verified by coulometric analysis. The sample location for each smear slide was entered into the J-CORES database with a sample code of SS. Figure F8 shows an example of the microscopic cuttings description.

Thin sections

Most sediment samples observed in cores were semiconsolidated; therefore, we decided to take only a few thin sections per core from a few pieces of conglomerate components (e.g., volcanic rocks) and cemented sedimentary rocks observed in split cores. Thin sections were prepared for more intensive analysis of the mineralogy, structure, and fabric. A 30 µm × 2 cm × 3 cm section of sediment was used for each thin section. Thin sections were polished and observed in transmitted light using a Zeiss Axioskop AX10 polarizing microscope equipped with a Nikon DS-Fi1 digital camera.

Mineralogical analysis of cuttings and core

X-ray diffraction

During Expedition 337, XRD analysis was used in conjunction with smear slides to provide a comprehensive, integrated approach to petrologic evaluation. The primary goal of XRD analyses was to identify relative mineralogical changes with depth of total clay, quartz, feldspar, and calcite based on peak areas identified in the diffraction patterns. XRD analysis of cuttings was conducted on 1–4 mm size fractions of selected washed cuttings samples, whereas core samples for XRD analysis were selected at a resolution of one per core from the working half and in the areas of lithologic change. Cuttings samples from 636.5 to 1263 m MSF were taken every 10 m for XRD measurements, whereas cuttings samples from 1263 to 2466 m MSF were analyzed every 40–50 m at the same intervals as samples for XRF. In some cases, the position of XRD samples was the same as for organic carbon, Rock-Eval pyrolysis, biomarker analysis, and calcium carbonate analyses.

Sample preparation included drying the samples for 24–72 h using a vacuum dryer (Laboconco FZ-4.5 CL) followed by crushing with a planetary ball mill (Fritsch P-5/4 Fritsch) at 200 rpm for 2 min. Very hard granules remaining in the samples after milling were removed. Bulk powdered samples were then mounted on a glass holder 24 (large size) and analyzed using a PANanalytical CubiX PRO (PW3800) diffractometer with the X-ray generator set to 45 kV and 40 mA. Scanning was conducted from 2°2θ to 60°2θ with a step size of 0.01° and sampling time of 1 s per step.

Peak areas were measured using MacDiff software for peaks associated with total clay (2θ = 19.301°–20.369°), quartz (2θ = 26.260°–26.957°), plagioclase (2θ = 27.463°–28.275°), and calcite (2θ = 28.887°–29.975°). Relative abundances were calculated using linear regression of the measured peak areas to known abundances in mixed mineral standards. Standards used for calibration were the same as those from ODP Leg 190 and IODP Expeditions 315, 319, 322, and 333 (Underwood et al., 2003; Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010a; Expedition 322 Scientists, 2010; Expedition 333 Scientists, 2012). This set of 14 standards contains illite, smectite, and chlorite clay (14%–76%), quartz (6%–57%), plagioclase feldspar (5%–38%), and calcite (2%–70%). Previous expeditions that used these standard mixtures sampled sediments from the Nankai Trough accretionary complex. The dominant minerals are broadly similar between the southern (e.g., Steurer and Underwood, 2003; Guo and Underwood, 2012) and northern (Mann and Müller, 1980; Kurnosov et al., 1980) sections of the Japan margin. Therefore, the mineral types and weight percent ranges within the standards developed for the Nankai Trough sediments encompass those from offshore the Shimokita Peninsula. These standards were also used for the Chikyu shakedown cruise in 2006 (Aoike, 2007). Although the linear regression calibration method quantifies the relative abundance of standard mineral mixtures with little error, actual sample measurements should be considered semiquantitative estimates of a four-component mixture. The presence of other constituents in marine sediments such as biogenic silica, volcanic glass, and lithic fragments can introduce additional error to relative abundances calculated using the calibration method. Qualitative identification of the minerals present in the XRD pattern was conducted using the PANalytical X’pert program, and the d-spacings of each peak were annotated automatically or manually using the International Center of Diffraction Data (www.icdd.com/) database. XRD results are available in XRD-CORE.XLSX, XRD-CUTTINGS.XLSX, and XRD-QUALITATIVE.XLSX in LITH in “Supplementary material.”

X-ray fluorescence

Core materials and cuttings were subjected to whole-rock quantitative XRF spectrometry for analysis of major elements (Na, Mg, Al, Si, Fe, P, K, Ca, Ti, and Mn). XRF analyses of all cuttings samples from Expedition 337 were conducted every 40–50 m at the same intervals as samples for XRD. In some cases, the position of XRF samples were the same as for total organic carbon (TOC), Rock-Eval pyrolysis, biomarker analysis, and calcium carbonate analysis using the 1–4 mm size fraction. On the other hand, XRF analyses of all core samples were done at a resolution of one per core and in areas of lithologic change at the same interval as samples for XRD analyses. All samples were dried and crushed before analysis, together with samples for XRD.

Approximately 1 g of sample powder was pressed into a pellet before analysis. Analyses were performed using a Supermini XRF spectrometer (Rigaku) with a 200 W Pd anode X-ray tube operated at 50 kV and 4 mA. Rock standards from the National Institute of Advanced Industrial Science and Technology were used for calibration of the XRF spectrometer, using matrix corrections within the operation software. Results were reported as weight percent oxide (Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, and Fe₂O₃). XRF results are available in XRF-CORE.XLSX and XRF-CUTTINGS.XLSX in LITH in “Supplementary material.”

X-ray CT scan

During Expedition 337, the assessment of core quality and identification of unique structural and sedimentological features, as well as determination of sampling locations, were facilitated using X-ray CT scanned images. Scanning was done immediately after splitting the core into sections and headspace gas sampling in the core cutting area. WRC sections were screened to avoid destructive sample processing of critical structural features.

The X-ray CT scan of cores was useful for the identification of 3-D sedimentary features, such as bioturbation burrows or bedding planes, for the estimation of lithology differences in areas without VCDs, and for the lithologic description of coaly horizons. Coal has a very low density in comparison to sandstone. Depending on the macrolithotypes and the purity of coal, the density values change and it is possible to estimate the composition of the coal in more detail. Concretions, isolated grains, and veins, which are often filled with pyrite, show very high density values and are clearly visible in the X-ray CT scan.

The X-ray CT instrument on the Chikyu is a GE Yokogawa Medical Systems LightSpeed Ultra 16 (GE Healthcare, 2006) capable of generating sixteen 0.625 mm thick slice images every 0.5 s, the time for one revolution of the X-ray source around the sample (Table T3). Data generated for each core consist of core-axis-normal planes of X-ray attenuation values with dimensions of 512 × 512 pixels. Data were stored as Digital Imaging and Communication in Medicine (DICOM) formatted files. The DICOM files were restructured to create 3-D images for further investigation.

The theory behind X-ray CT has been well established through medical research and is very briefly outlined here. X-ray intensity varies as a function of X-ray path length and the linear attenuation coefficient (LAC) of the target material as

where

• I = transmitted X-ray intensity,

• I₀ = initial X-ray intensity,

• η = LAC of the target material, and

• L = X-ray path length through the material.

LAC is a function of the chemical composition and density of the target material. The basic measure of attenuation, or radiodensity, is the CT number given in Hounsfield units (HU) and is defined as

where

• η_t = LAC for the target material, and

• η_w = LAC for water.

The distribution of attenuation values mapped to an individual slice comprises the raw data that are used for subsequent image processing. Successive 2-D slices yield a representation of attenuation values in 3-D pixels referred to as voxels.

Analytical standards used during Expedition 337 were air (CT number = –1000), water (CT number = 0), and aluminum (2477 < CT number < 2487) in an acrylic core mock-up. All three standards were run once daily after air calibration. For each standard analysis, the CT number was determined for a 24.85 mm² area at fixed coordinates near the center of the cylinder.

Scanning electron microscopy

Scanning electron microscopy and energy dispersive spectroscopy using a JEOL electron microscope were used for a detailed observation and detection of minerals of a few selected samples of interest, principally the coal and carbonate-rich layers.

Identification of lithology

Lithologic units and boundaries were identified with core and cuttings analyses (i.e., VCD, smear slides, XRD, and XRF), logging data, and physical properties measurements. Compositional and textural features were recorded mainly based on the archive halves of cores. As the size of cuttings was generally small, it was not possible to use cuttings for the determination of sedimentary structure attributes, whereas X-ray CT scan and multisensor core logger (MSCL) data were useful for lithostratigraphic description. WRC samples that were taken for microbiology (MBIO), interstitial water (IW), or shore-based analyses were not available for core description or the MSCL-I. However, scraped sample portions and residues of MBIO and IW samples were described using the smear slide method.

Downhole log response is a good tool to support the identification of lithostratigraphic units, transitions, and boundaries. Especially in noncoring depth intervals, data from borehole logging and cuttings were extremely important for good interpretation of lithostratigraphic units. For this purpose, we used gamma ray, sonic, electrical borehole image, and resistivity logs (from 1263 meters below seafloor [mbsf] to the bottom of the hole; see “Downhole logging”).

The integration of all available data (e.g., cuttings, cores, X-ray CT scan, MSCL, logging data, and physical properties measurements) was used for the identification of the lithostratigraphic characterization, interpretation of geological features (e.g., transitions, sequences, and boundaries), and identification of the composition and physical properties within each unit.

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