Previous   |    Next

doi:10.2204/iodp.proc.314315316.122.2009

Lithology

Visual core descriptions

We followed conventional Ocean Drilling Program (ODP) and IODP procedures for recording sedimentologic 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 shipboard database software J-CORES and then converted to core-scale depictions using Strater 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).

The Graphic Lithology column on each VCD plots to scale all beds that are at least 2 cm thick. Graphic patterns do not show persistent interlayers <2 cm thick, but such intervals are identified as laminae in the Sedimentary Structures column. It is difficult to discriminate between the dominant lithologies of silty clay and clayey silt without quantitative grain size analysis, so we grouped this entire range of textures into the category “silty clay” on all illustrations. 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 F4 displays graphic patterns for all lithologies encountered during Expedition 315. Also shown are symbols for internal sedimentary structures, soft-sediment deformation structures, and severity of core disturbance in both soft sediment and indurated sedimentary rock.

Smear slides

Smear slides are useful for identifying and reporting basic sediment attributes (texture and composition), but the results are not quantitative. We estimated the abundances of biogenic, volcaniclastic, and silicilclastic constituents with the help of a visual comparison chart (Rothwell, 1989). Errors can be large, however, especially for fine silt and clay-size fractions, and reproducibility among different sedimentologists is poor. Smear slide analysis also tends to underestimate the amount of sand-size grains because they are difficult to incorporate evenly onto the slide. Thus, it would be misleading to report values as absolute percentages. Instead, our descriptive results are tabulated as visual percentage estimates, with values grouped into the following range categories:

• D = dominant (>50%).
• A = abundant (>20%–50%).
• C = common (>5%–20%).
• P = present (>1%–5%).
• R = rare (0.1%–1%).
• T = trace (<0.1%).

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

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 position of each specimen is shown on the VCD slide editor column of the VCD application. We tabulated data in an Excel spreadsheet because data entry into J-CORES is prohibitively time-consuming and the program will not accept ranges of values for individual compositional categories.

X-ray diffraction

We completed routine XRD analyses of bulk powders using a PANalytical CubiX PRO (PW3800) diffractometer. Our principal goal was to estimate relative weight percentages of total clay minerals, quartz, plagioclase, and calcite using peak areas. Most of the samples were selected from intervals adjacent to whole-round samples, and most are part of sampling clusters with physical properties and carbonate. A few additional samples were collected periodically from such unusual lithologies as carbonate-cemented claystone and volcanic ash. Samples were freeze-dried, crushed with a ball mill, and mounted as random bulk powders. The instrument settings were as follows:

• Generator = 45 kV and 40 mA.
• Tube anode = Cu.
• Wavelength = 1.54060 Å (K_α1) and 1.54443 Å (K_α2).
• Step spacing = 0.005°2θ.
• Scan step time = 0.648 s.
• Divergent slit = automatic.
• Irradiated length = 10 mm.
• Scanning range = 2°2θ to 60°2θ.
• Spinning = yes.

In order for our results to match those of ODP Leg 190 as closely as possible, the choice was made to use MacDiff version 4.2.5 software (www.ccp14.ac.uk/​ccp/​ccp14/​ftp-mirror/​krumm/​Software/​macintosh/​macdiff/​MacDiff.html) for data processing. Each peak’s upper and lower limits were adjusted following the guidelines shown in Table T3. Calculations of relative mineral abundance utilized a matrix of normalization factors derived from integrated peak areas and singular value decomposition (SVD) (Table T4). 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 Expedition 315 after the diffractometer’s high-voltage power supply was replaced. Bulk powder mixtures for the Nankai Trough are the same as those reported by Underwood et al. (2003): quartz (Saint Peter sandstone), feldspar (Ca-rich albite), 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 F5.

Average errors (SVD-derived estimates versus true weight percent) are total clay minerals = 3.5%, quartz = 2.1%, plagioclase = 0.8%, and calcite = 1.5%. In spite of its precision with standard mixtures, the SVD method is only semiquantitative and results for natural specimens should be interpreted with some 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 plagioclase). Contents of clay minerals are best characterized by measuring the peak area, whereas peak intensity may be easier and more accurate to quantify quartz, feldspar, and calcite. Analyzing oriented aggregates enhances basal reflections of the clay minerals, but this step is time consuming and requires isolation of the clay-size fraction to be effective. Errors also propagate as more minerals and peaks are added to the procedure. For clay mineral assemblages, 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° to 20.4°2θ. Another source of error is contamination of mineral standards by impurities such as quartz (e.g., the illite standard contains ~20% quartz). 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 values of a mineral’s weight percent should only be regarded as relative percentages within a four-component system of clay minerals + quartz + plagioclase + calcite. How close those estimates are to their absolute percentages within the mass of total solids will depend 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 absolute errors are probably between 5% and 10%. Thus, the primary value of bulk powder XRD data should be to identify spatial and temporal trends in sediment composition and to assist with core-log integration.

Top of page   |    Previous   |    Next