- Chapter contents
- Abstract
- Introduction
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Methods and materials -
Results
- Acknowledgments
- References
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Figures
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Tables
- Appendix
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Appendix figures
- PDF file
doi:10.2204/iodp.proc.348.201.2017
Results
Textural and compositional characterization
Shipboard scientists used visual inspection to classify the dominant lithology of WR core samples as silty claystone to clayey siltstone. One sample (348-C0002P-4R-2, 98 cm) contains a clearly defined bed of siltstone (Fig. F3), and millimeter-scale heterogeneities also exist in the other specimens. We used SEM and EDS to quantify the differences in grain size and characterize minor compositional constituents (e.g., pyrite) in greater detail (Fig. F8). Additional images are included in the Appendix (Figs. AF4, AF5, AF6, AF7, AF8, and AF9). Figures AF5 and AF9, for example, provide EDS documentation for specimens containing carbonate microfossils and authigenic barite.
Figure F9 shows a range of grain sizes in a specimen that was cut with random orientation from Sample 348-C0002P-3R-1, 51 cm. Part of the image consists of coarse silt–size and fine sand–size grains (apparent diameters = ~50–100 µm). Medium silt–size grains (~20–40 µm) dominate the other part. The coarser material also displays a greenish color (Fig. F9A).
Pyrite (FeS) is a common authigenic mineral in deep-sea sediments (e.g., Berner, 1984) and has been noted in other specimens from the Nankai Trough (e.g., Egawa et al., 2015). Figure F10 shows pyrite framboids in a specimen that was cut from Sample 3R-1, 51 cm. The high counts of S and Fe match the black band identified from the shipboard visual descriptions and an X-ray computed tomography scan of the WR sample. Small amounts of pyrite also occur in the specimen cut from Sample 4R-2, 98 cm. The individual particles are ~1 µm and collectively form spheroidal aggregates.
Figure F11 shows SEM images from Sample 348-C0002P-4R-2, 98 cm. The trimmed sample contains a prominent bed of medium siltstone oriented oblique to the core axis (Fig. F3). That layer is dominated by grains measuring ~25 µm (Fig. F11B). Smaller grains in the surrounding clayey siltstone are ~12 µm in apparent diameter. Figure F12 shows SEM images from Sample 348-C0002P-6R-1, 64 cm. That clayey siltstone sample is comparatively homogeneous, and no obvious differences in grain size exist among the imaged portions of the specimen. The dominant particle size for that material is ~12–15 µm.
Microfabric statistics
Figure F13 catalogs the rose diagrams of particle orientation and corresponding values of standard deviation and index of orientation (see Table T2 for statistics). The cumulative frequency curves are plotted on Figure F14, which shows the orthogonal pairs of ESEM images for specimens tested for permeability. In general, we see no visual evidence for preferred orientation of grains, particularly for specimens with higher contents of silt. As statistical confirmation, the standard deviation for grain orientation ranges from 53.6° to 58.7°, and the index of orientation ranges from 0.19 to 0.26. All such values are consistent with random arrangements of particles. The index of orientation increases slightly with depth, especially for measurements on the vertical cut face (Fig. F15).
Permeability
Table T1 summarizes values of water content and porosity for trimmings measured before and after each flow-through test. The trimmings are not necessarily representative of the entire cylinder, especially for specimens containing pyrite bands and coarser siltstone laminae. Two sections (348-C0002P-4R-2 and 6R-1) yielded pretest values of water content lower than the shipboard values. Those differences probably were caused by moisture loss during the shipment and storage of the WR specimens. In addition, we note that trimmings dried out quickly during the sample preparation. For Section 4R-2, the pretest porosity is also lower than post-test porosity. That specimen was particularly indurated, so moisture was probably lost during the longer time required for trimming. For Sample 348-C0002P-3R-1, 57 cm, the shipboard value of porosity is lower than either pretest or post-test porosity. Moisture may have been gained during the trimming procedure; a spray bottle was used to keep the sample saturated during trimming. Alternatively, the higher post-test value may be related to progressive expansion of water-filled microcracks after unloading.
An illustration of each individual constant-flow test result is included in the Appendix (Figs. AF1, AF2, AF3). With one exception, we regard the test results as “reliable” because the coefficient of determination (R2) is >0.9835. The exception (Section 348-C0002P-6R-1, at 0.28 MPa) yielded R2 = 0.9791, and that specimen was probably compromised by microcracks; subtle cracks were observed during trimming. In addition, the flow response was notably erratic under the higher effective stress (0.55 MPa). While trying to trim a second specimen, the material fragmented along those crack lines. Our primary goal was to test specimens with different textures, especially coarser turbidite layers. That goal was achieved by successfully testing Section 348-C0002P-4R-2, with its intact interbeds of siltstone and clayey siltstone.
Table T3 lists the average values of hydraulic conductivity and intrinsic permeability for each specimen. Table T4 lists values of volumetric flow rate, discharge velocity, steady-state head loss, steady-state hydraulic gradient, hydraulic conductivity, and intrinsic permeability for each test. The highest average value of vertical hydraulic conductivity (K) is 2.68 × 10–8 cm/s, with k = 2.66 × 10–17 m2. The lowest average value of K is 3.83 × 10–9 cm/s, with k = 3.80 × 10–18 m2. Figure F16 shows a comparison between values of vertical permeability from this study and data from other NanTroSEIZE sites along the Kumano transect, plotted as a function of burial depth. This compilation includes data from both flow-through tests and constant rate of strain (CRS) consolidation tests, with values of effective stress ranging from 0.02 to 12.4 MPa. In general, permeability values for mudstones decrease with depth, although the scatter at any given depth extends over 5 orders of magnitude. Permeability values for shallow Site C0002 mudstones plot near the center of this range. The three deeper samples from Hole C0002P, including the coarser specimen from Section 4R-2, show no significant differences in permeability with respect to shallow mudstones at Site C0002.
Figure F17 displays the relation between permeability and post-test porosity for a compilation of mud and mudstone data from different subduction zones. The values of porosity have not been corrected for contents of smectite (i.e., the effects of interlayer water). The global trend shows a systematic decrease in permeability with decreases in porosity. Shallow mudstone samples from Site C0002 yield permeability values near the middle of the compilation, over a comparable range of 40% to 50% porosity. Permeability values from the deeper and less porous Hole C0002P samples are similar in order of magnitude to the results from shallow Site C0002 mudstone.