Previous   |   Next

doi:10.2204/iodp.proc.319.102.2010

Lithology

At Site C0009, cuttings, wireline logging data, gamma ray data from MWD, and a limited amount of core were available to identify lithologic units. At Sites C0010 and C0011, only a limited suite of LWD and MWD data were available, including gamma ray and RAB.

Cuttings and core samples at Site C0009 were described based on

• Macroscopic observations,

• Microscopic observations (including smear slides and thin sections),

• Mineralogical analysis (XRD and XRF), and

• Grain-size separation (Atterberg method).

Macroscopic observations of cuttings and core

Macroscopic observations of cuttings

Cuttings are small fragments of rocks, in general 0.25–4 mm in size, of various lithologies produced during drilling. Cuttings were taken for the first time in IODP operations during Expedition 319. These solid fragments are suspended in drilling mud that contains dissolved clay from the formation, as well as small amounts of claylike drilling additives (e.g., bentonite), which hampers the quantification of the true clay content. Drilling mud also contains disaggregated rock and sediment fragments that were also analyzed. The separation procedure of cuttings from the clay mud and the division into different sizes is explained in "Introduction."

Cuttings were taken every 5 m from 703.9 to 1592.7 m MSF, and a second set of cuttings was collected together with core from 1507.7 to 1603.7 m MSF with the same sampling interval (199 samples in total). Based on macroscopic observations of the unwashed bulk material, we estimated the relative amount of coarser grained (e.g., sand/silt) and finer grained (e.g., clay) materials, the induration state of the bulk material, the occurrence of contamination, and the presence of wood fragments, as well as the appearance, softness, relative amount, size, and degree of lithification of hard rock fragments. We defined cuttings from

• 707.7 to 802.7 m (Samples 319-C0009A-SMW-3 through SMW-24) as mud,

• 802.7 to 1037.7 m (Samples 319-C0009A-SMW-27 through SMW-128) as disaggregated mudstone, and

• 1037.7 to 1603.7 m (Samples 319-C0009A-SMW-128 through SMW-215) as mudstone.

All macroscopic observations were recorded on visual cuttings description forms and summarized in C0009_T1.XLS in LITHOLOGY in "Supplementary material."

Macroscopic observations of core

We followed conventional Ocean Drilling Program (ODP) and IODP procedures for recording sedimentological information on VCD forms on a section-by-section basis (Mazzullo and Graham, 1988). Core descriptions were transferred to section-scale templates using J-CORES. 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 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, so we grouped this entire range of textures into the category "silty claystone" 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. Claystone differs from silty claystone and coarser grained sediment by a smoother cut surface. Figure F8 displays graphic patterns for all lithologies encountered during Expedition 319. 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.

Microscopic observation of cuttings

A 30 cm³ aliquot of bulk cuttings was taken for sedimentological description, and smear slides were made in order to identify major lithologic changes. Microscopic investigations of the washed >45 µm (silt and sand size) fraction 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 Figures F17 and F18 in the "Site C0009" chapter and in C0009_T2.XLS in LITHOLOGY in "Supplementary material." With this technique, the clay fraction could not be evaluated. It is also not possible to discriminate between sand grains that originally formed from uncemented or weakly cemented sand horizons and grains that were originally dispersed in finer grained horizons.

Microscopic observation of core

The microscopic description of core samples follows the same principles as for cuttings. Here, several very small rock chips were carefully taken with a small squeezer from different areas of the archive core for observations under the binocular microscope. The sample description includes an estimation of mineral occurrence and abundance, as well as grain roundness and sorting.

Smear slides

Smear slides are useful for identifying and reporting basic sediment attributes (texture and composition) in samples of both cuttings and cores, but the results are not quantitative. We estimated the abundance of biogenic, volcaniclastic, and siliciclastic constituents using a visual comparison chart (Rothwell, 1989). Errors can be large, however, 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%).

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

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. Sediments, cuttings, and rocks that were altered, badly weathered, or contained a 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 Pertopoxy 154. After polishing the samples, thin sections were covered by a cover glass with Canada balsam. Thin sections were observed in transmitted light using an Axioskop 40A polarizing microscope (Carl Zeiss) equipped with a Nikon DS-Fi1 digital camera.

Mineralogical analysis of cuttings and core

X-ray diffraction

The principal goal of XRD analysis of both cuttings and core 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 1–4 mm size fractions of washed Samples 319-C0009A-76-SMW through 194-SMW (see "Introduction" for details of sample preparation). Further grain size separation was performed to analyze the clay mineral composition. Some core samples were selected for XRD analysis from intervals adjacent to whole-round samples. Most were part of sampling clusters taken for physical property and carbonate analyses, which were taken once per core section and in areas of lithologic changes. All samples were freeze-dried, crushed with a ball mill, and mounted as randomly oriented bulk powders.

We completed routine XRD analyses of bulk powders from cuttings and core using a PANalytical CubiX PRO (PW3800) diffractometer. XRD instrument settings were as follows:

• Generator = 45 kV.

• Current = 40 mA.

• Tube anode = Cu.

• Wavelength = 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 T6. 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 Expedition 315 after the diffractometer's high-voltage power supply and X-ray tube were replaced (Ashi et al., 2008), and the mixtures were rerun at the beginning of Expedition 319 (Table T7). 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 F9.

Average errors (SVD-derived estimates versus true weight percent) of the standard mineral mixtures are total clay minerals = 3.3%, quartz = 2.1%, plagioclase = 1.4%, and calcite = 1.9%. 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). 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 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 fluids. The severity of these artifacts is especially obvious in the calculated values of percent calcite. We suspect this is a result of rapid precipitation of calcium carbonate from high-ph fluid coming from the drilling mud, although geochemistry data show a relatively constant pH of >10 during drilling (see "Geochemistry"), which cannot completely explain variations in calcite content downhole (see "Lithology" in the "Site C0009" chapter).

Clay mineral analysis

We also separated three different grain-size fractions from five bulk rock samples by sieving and centrifuging (>63 µm, 2–63 µm, and <2 µm). To calculate the rotation speed (rpm) and the running time (min) of the centrifuge, we used the SediTools software (R. Petschick, Geol. Palaeontolog. Institute, University of Frankfurt, Germany). The values were corrected for acceleration and deceleration times.

After 1–2 min of ultrasonic treatment in deionized water, we wet-sieved the samples in a 63 µm sieve to separate the sand/silt grain size from the clay fraction. The >63 µm fraction was dried in an oven at 50°C, and the remaining suspension was used for separating the <2 µm fraction. After another ultrasonic treatment for ~1–2 min, the samples were centrifuged for 3 min at 1000 rpm. The suspension contained the <2 µm grain-size fraction, and the solid material contained the 2–63 µm grain sizes. All samples were then oven-dried at 50°C, and ~0.5 mg of the resulting powder was mixed with ~1.5 mL of deionized water, treated ultrasonically for ~1 min, and placed on an XRD glass sample holder with a pipette. After drying, the samples were analyzed by XRD.

X-ray fluorescence

We performed whole-rock quantitative XRF spectrometry analysis for major elements on cuttings and on core material. The 1–4 mm size fraction of cuttings samples was used for these measurements. Samples of 10 cm³ were taken from the working half of the core next to samples for XRD analysis. 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 T8. 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 T9 lists the results and standard deviations 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 J-CORES and are shown in Figure F20 in the "Site C0009" chapter (see also C0009_T4.XLS in LITHOLOGY in "Supplementary material").

Grain size separation (Atterberg method)

To estimate the sand (>63 µm), silt (2–63 µm), and clay (<2 µm) fraction in the cuttings, we separated these fractions by sieving and settling methods (Atterberg method). The conventional Atterberg method is used for separating grain size fractions according to their settling velocity. After the sample was poured into a sedimentation cylinder, deionized water was added up to the desired settling height. The closed cylinder was shaken until the suspension was homogeneous. When the necessary settling time for a given equivalent diameter (e.g., 2 µm) was reached (calculated according to Stokes law), the supernatant suspension (e.g., only material <2 µm) was decanted and dried. In order to achieve optimal separations, this procedure should be repeated up to 15 times. However, because of restricted time, it was repeated only three times during this expedition.

We note that the percentage of "total clay" of the 1–4 mm fraction measured by XRD (45 wt%) is higher than the clay-size fraction determined by the Atterberg method on unwashed cuttings (24%). Our preliminary interpretation is that samples used in Atterberg analysis contained nondisaggregated agglomerates of silty clay that were not disaggregated during shaking, leading to underestimation of the fine-grained fraction. Mixing with drilling mud may also contribute to this effect.

Identification of lithologic units

At Site C0009, we used logging data in conjunction with analyses of core and cuttings to identify lithologic units and boundaries. We identified compositional and textural attributes of the formation mainly using nuclear (gamma ray, density, and PEF) and sonic logs along with data from cuttings and core where available. 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, natural- and induced-radioactivity logs, 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 likely lithologic composition). These data are shown in Figure F19 in the "Site C0009" chapter.

At riserless Sites C0010 and C0011, we collected a limited suite of LWD/MWD data (gamma ray and RAB, including resistivity images); operations at these sites did not include recovery of core or cuttings. At Site C0010 we identified the major units and boundaries using gamma ray and resistivity data and by comparison with previously drilled nearby IODP Site C0004 (3.5 km to the northeast along strike), where a more extensive suite of LWD data and coring were used to define logging and lithologic units during IODP Expeditions 314 and 316 (Kinoshita et al., 2008; Kimura et al., 2008).

Top of page   |   Previous   |   Next