DISCUSSION

Sedimentary structures such as various types of lamination can be clearly to faintly recognized in X-ray images. This implies that the sedimentary rock (sandstone, siltstone, mudstone, and shale) retains density contrasts in spite of the decrease in porosity contrasts caused by compaction. Grain density and porosity profiles (Shipboard Scientific Party, 2004b; fig. F159) show that grain density is consistent throughout the hole and that sandstone and grainstone often have porosities as much as 20% lower than the general trend. Therefore, the density contrast in lamination, which generally consists of repetition of sand-sized grain layers and clay-silt–sized grain layers, is considered to be mainly attributed to remnant of porosity contrast, whereas some sedimentary structures are invisible in X-ray images. This invisibility may be induced by several factors:

1. Resolution of the X-ray image: The threshold thickness of visible/invisible sedimentary structures on 2-D X-ray images is greater than ~0.5 mm (Table T1). The minimum thickness of 0.5 mm (Fig. F7D) seems to represent the practical resolution of 2-D images produced by the portable X-ray CT system. In many cases, the minimum thicknesses of sedimentary structures of interest are ~1 mm. Integration of the images across the entire thickness of a core may make the resolution of the X-ray image coarser than the ideal resolution.
2. Whole-rock cementation: Table T1 shows that sedimentary structures in grainstone, marlstone, and calcareous sandstone-siltstone are often invisible on CT images even if the structures are >1 mm thick. The common characteristic of these rocks is that they are cemented by carbonate minerals. Therefore, carbonate cementation may render density distributions that reflect sedimentary structures homogeneous, preventing clear imaging of these sedimentary structures. CT imaging may be more suitable for observation of sedimentary structures in cemented rocks by selecting narrower ranges of CT processing values.
3. Direction of image: Three-dimensional observation of trough cross-lamination from various directions (Figs. F10, F11) shows that side views of the some lamina sets are distinct but at ~90° rotation from the side view laminae are unclear because of overlap. Images of soft-sediment deformation may also be influenced by direction effect. In several cases, intrabed slumps are invisible in 2-D X-ray images (Fig. F5B, F5D) but are clearly observed in another 2-D X-ray image (Fig. F6B). Like cases of high-angle trough cross-laminae, overlap of folded laminae seems to prevent the clear appearance of soft-sediment deformation structures.

On the other hand, X-ray images can reveal sedimentary and postsedimentation structures such as burrows, water escape structures, and small concretions that cannot be seen on the split core surface (Figs. F7, F8, F9, respectively). These structures tends to be filled with authigenic minerals such as calcite and pyrite.