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doi:10.2204/iodp.proc.318.102.2011

Lithostratigraphy

This section outlines the procedures for documenting the sedimentology of deposits recovered during Expedition 318, including core description, smear slide description, color spectrophotometry, and XRD of clay mineral preparations. Only general procedures are outlined. All data were uploaded into the IODP-USIO LIMS database and observations were entered using the DESClogik application in Tabular Data Capture mode. DESClogik also includes a graphic display mode for core data (e.g., digital images of section halves and measurement data) that was used for quality control of the uploaded data sets.

Visual core descriptions

Information from macroscopic and smear slide description of each core was recorded manually on visual core description (VCD) forms. Standard sedimentological observations of lithology, sedimentary structures, bioturbation, and diagenesis were made. Entry of descriptive information using the DESClogik program was performed through the Tabular Data Capture mode. Scanned VCD forms were also entered into DESClogik. A template was constructed and tabs and columns were customized to include relevant descriptive information categories (lithology, sedimentary structures, macrofossils, bioturbation, diagenesis, drilling disturbance, clast properties, and clast abundance). A summary description was also entered for each core.

Standard graphic report (barrel sheet)

A simplified one-page graphical representation of each core (barrel sheet) was generated using the LIMS2Excel application and a commercial program (Strater, Golden Software). Barrel sheets are presented with the CSF depth scale, split-core photographs, graphic lithology, and columns for core disturbance, clast abundance, sedimentary structures, macrofossils, diagenesis, bioturbation, shipboard samples, magnetic susceptibility, color reflectance (b*), and GRA density. The graphic lithologies, sedimentary structures, and other visual observations are represented on the barrel sheets by graphic patterns and symbols (Fig. F2). Each barrel sheet also contains the summary description for the core.

Smear slides

Smear slide microscopic analysis was used to determine microfossil constituents and abundance to aid in lithologic classification. Toothpick samples were taken in each lithology and at a frequency of at least one sample every other section (~3 m). For these preparations, the sediment was mixed with distilled water on a glass coverslip and dried on a hot plate at 50°C. The dried sample was then mounted in Norland optical adhesive 61 and fixed in an ultraviolet light box. Type and relative abundance of biogenic and mineralogic components were estimated for each smear slide. Data were entered into LIMS using a custom tabular template in DESClogik.

Lithologic classification scheme

Lithologic terminology for granular sediments and rocks was based on a combination of the classification systems used during ODP Leg 188 and ANDRILL projects (Shipboard Scientific Party, 2001; Naish et al., 2006).

Principal terminology

The principal lithologic name was assigned on the basis of the relative abundances of pelagic biogenic and terrigenous clastic grains.

The principal name of a sediment/rock with <50% pelagic biogenic grains was based on the grain-size characteristics of the terrigenous clastic fraction:

• If the sediment/rocks contain no gravel, then the principal name was determined by the relative abundances of sand, silt, and clay (Fig. F3; after Mazzullo et al.,1988).

• If the sediment/rocks contain terrigenous clastic gravel, then the principal name was determined by the abundance of gravel and the sand/mud ratio of the terrigenous clastic matrix (Fig. F4). This scheme was modified after Moncrieff (1989) to include elements of the Mazzullo et al. (1988) classifications.

The principal name of a sediment/rock with >50% pelagic biogenic grains was classified as an “ooze,” modified by the most abundant specific biogenic grain type that forms 50% or more of the sediment/rock (e.g., if diatoms exceed 50%, then the sediment was classified as a “diatom ooze”). However, similar biogenic grain types were grouped together to exceed this 50% abundance threshold (e.g., if diatoms are 40% of the sediment and sponge spicules are 20%, then the sediment was termed “biosiliceous ooze”) (Fig. F5).

Major and minor modifiers were applied to any of the principal granular sediment/rock names. The use of major and minor modifiers follows the scheme of ODP Leg 188 (Shipboard Scientific Party, 2001):

• Major modifiers are those components with abundances between 25% and 50% and are indicated by the suffix “-rich” (e.g., “diatom-rich”).

• Minor modifiers are those components with abundances of 10%–25% and are indicated by the suffix “-bearing” (e.g., “diatom-bearing”).

• If possible, modifiers were assigned on the basis of the most abundant specific grain type (e.g., “silt-rich” or “silt-bearing”).

The Wentworth (1922) scale was used to define grain-size classes. For units with >1% gravel, estimated using the comparison chart of Terry and Chilingar (1955), lithology was defined using the modified Moncrieff (1989) classification scheme.

Clast abundance and properties

Counts were made of the total number of clasts larger than 2 mm in every 10 cm of core. For clasts exceeding 2 cm in width, the lithology, rounding, and surface texture of the clasts was noted separately on the visual core description sheets.

Bioturbation

Ichnofabric description analysis included evaluation of the extent of bioturbation and notation of distinctive biogenic structures. To assess the degree of bioturbation semiquantitatively, the ichnofabric index from Bann et al. (2008) (from 1 to 6) was employed (e.g., 1 = bioturbation absent, 3 = moderate bioturbation, and 6 = total biogenic homogenization of sediment). This index is illustrated using the numerical scale in the Relative Bioturbation column of the barrel sheets. Recognizable biogenic structures and trace fossils were noted and logged in the database.

Core disturbance

Drilling disturbance was characterized in the barrel sheets with the symbols given in Figure F2. Drilling disturbance of relatively soft or firm sediments (i.e., where intergrain motion was possible) was classified into four categories:

1. Slightly disturbed: bedding contacts are slightly bent.

2. Moderately disturbed: bedding contacts are extremely bowed.

3. Extremely disturbed: bedding is completely deformed and may show diapiric or minor flow structures.

4. Soupy: sediments are water saturated and show no traces of original bedding or structure.

Drilling disturbance of lithified sediments (i.e., wherein intergrain motion was not likely because of compaction, cementation, etc.) was classified into five categories:

1. Slightly fractured: core pieces are in place and have very little drilling slurry or brecciation.

2. Moderately fractured: broken core pieces are in place or partly displaced, but original orientation is preserved or recognizable.

3. Moderately fractured or biscuited: core pieces are from the cored interval and are probably in correct stratigraphic sequence (although they may not represent the entire section); intact core pieces are broken into rotated discs (or “biscuits”) as a result of the drilling process.

4. Highly fractured or brecciated: pieces are from the cored interval and are probably in the correct stratigraphic sequence (although they may not represent the entire section), but original orientation is totally lost.

5. Highly fractured or drilling slurry: pieces are from the cored interval and are probably in the correct stratigraphic sequence (although they may not represent the entire section), but original orientation is totally lost; loose pieces of core material are mixed with drilling slurry.

In addition to these main categories for soft and lithified sediments, several other terms were used to characterize drilling disturbance:

• Washed gravel: fine material is suspected to be lost during drilling, with only washed coarse material remaining. This may have resulted from problems in the drilling and recovery of coarse-grained lithologies.

• Flow-in: soupy, displaced sediment pulled into the core liner during retrieval.

• Fall-in: downhole contamination resulting from loose materials falling from the drill hole walls.

• Sand/gravel contamination along core liner: isolated pieces of coarse contamination occurring alongside the core liner away from the core top.

Digital color imaging

The SHIL captures continuous high-resolution images of the archive-half surface for analysis and description. The instrument was used shortly after core splitting in an effort to avoid time-dependent color changes resulting from sediment drying and oxidation. The shipboard system uses a commercial line-scan camera lens (AF Micro Nikkon; 60 mm; 1:2.8 D), with illumination provided by a custom assembly of three pairs of light-emitting diode strip lights that provide constant illumination over a range of surface elevations. Each pair of lights has a color temperature of 6,500 K and emits 90,000 lux at 3 inches. The resolution of the line-scan camera was set at 10 pixels/mm. Users set a crop rectangle for each image to remove extraneous information. Images were saved as high-resolution TIFF files. Available files include the original high-resolution image with grayscale and ruler, as well as reduced JPEG images cropped to show only the section-half surfaces.

Spectrophotometry and colorimetry

The SHMSL employs multiple sensors for the measurement of bulk physical properties in a motorized and computer-controlled section-half logging machine. The sensors included in the SHMSL are reflectance spectroscopy and colorimetry, magnetic susceptibility, and a laser surface analyzer. Reflectance spectroscopy (spectrophotometry) was carried out using an Ocean Optics USB4000 spectrophotometer. This instrument measures the reflectance spectra of the split core from the ultraviolet to near-infrared range. Colorimetric information from split cores is also recorded by this instrument in the L*a*b* color space system. The L*a*b* color space expresses color as a function of lightness (L*) and color values a* and b*, where a* reflects the balance between red (positive a*) and green (negative a*) and b* reflects the balance between yellow (positive b*) and blue (negative b*). When a* and b* are 0, there is no color and L* determines grayscale.

Accurate spectrophotometry using the SHMSL demands a flush contact between the instrument sensors and the split core. A built-in laser surface analyzer aids the recognition of irregularities in the split core surface (e.g., cracks and voids), and data from this tool were recorded in order to provide an independent check on the fidelity of SHMSL measurements.

X-ray diffraction analysis

Selected samples for XRD analysis were obtained from the working halves of the cores at an average spacing of one sample per core. XRD analysis was performed on the clay fraction in most samples. For these preparations, a ~2 g sample was placed in a 50 mL centrifuge tube with 10% acetic acid, sonicated for 15 min, and allowed to stand overnight to remove carbonate material. After centrifuging for 15 min at 1500 rpm, the acetic acid was decanted, 25 mL of distilled water was added, the sample was centrifuged again, and the water was decanted. This washing procedure was repeated two more times to remove both the acid and salts from the sample. After decanting the final wash, 25 mL of 1% sodium metaphosphate solution was added to the sample in a 50 mL beaker. The sample was then placed in a ultrasonic bath for 5 min to suspend the clays by ultrasonic disaggregation and then centrifuged for 5 min at 1000 rpm to settle the >2 µm particles. The clays that remained in suspension were removed from the top ~1 cm of the centrifuge tube and pipetted onto two amorphous quartz sample discs. The sample discs were then left to air dry in a desiccator. After drying, one disc was analyzed and the other solvated with ethylene glycol for ~8 h at 65°C and reanalyzed to determine the presence of expandable clays.

The prepared samples were mounted onto a sample holder and analyzed by XRD using a Bruker D-4 Endeavor diffractometer mounted with a Vantec-1 detector using nickel filtered CuKα radiation. The standard locked coupled scan was as follows:

• Voltage = 35 Kv.

• Current = 40 mA.

• Goniometer scan = 3.5° to 30°2θ.

• Step size = ~0.0085°.

• Scan speed = 1 s/step.

• Divergence slit = 0.3°.

The diffractograms of single samples were evaluated with the Bruker Diffrac-Plus EVA software package. Relative abundances of the major clay mineral groups were established on the basis of maximum peak intensity, preferentially from the glycolated analysis. Quantification of mineral contents was not possible, as the samples were not spiked with a defined amount of a mineral standard for calibration. Therefore, the shipboard results were interpreted qualitatively on the basis of relative occurrences and abundances of the most common clay mineralogical components.

A small number of selected samples were freeze-dried, ground, and mounted on aluminum holders for bulk XRD analysis. Scans of these samples were performed with the same instrument settings as the clay preparations and scanned over a goniometer range of 3.5° to 70°2θ.

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