## 4 Results
### Microstructural Analysis
#### 4.2.1 Microstructure
Further, authigenic clay minerals, which were identified as smectite, grew in pore spaces created by fracturing in deformation bands (Table 3, Figure 5e). Figure 5: Microphotographs of deformation bands in borehole core sample B6. (**a**) Whole thin section (plane polarized light), where deformation bands and host areas are divided by red dashed lines. See Figure 3c for the locality. (**b**) Microstructures of coal seams, deformation bands (DB) and host areas (H) (plane-polarized light). (**c**) Alignment of fine-grained biotite grains in deformation bands shown by white dashed lines (crossed polarized light). OCL, orientation of coal layer in (**a**). (**d**) Extended and kinked biotite grains (crossed polarized light, shown by a white arrow). (**e**) Authigenic smectite grains grown in pore spaces created by fracturing of detrital grains. Measurements points for microchemical analysis with EPMA are indicated by filled red circles. See Table 3 for the compositions. See (**a**) for the localities of (**b–e**). The up direction of the borehole is shown by arrows in the diagrams (**a–d**). Qz, quartz; PI, plagioclase; Bt, biotite; Sme, smectite. In the fine-grained sandstone from outcrop sample KT1 of the Shakubetsu Formation, although no cataclasic bands developed, detrital biotite grains were aligned, forming phyllosilicate bands (Figure 6a). These detrital biotite grains were bent (kinked) by sliding along cleavages as they occurred in the borehole core sample B6_8.2 m. Furthermore, the SEM-BSE photomicrograph revealed that not only detrital biotite grains were altered to vermiculite and further to kaolinite, but smectite grew in the pore space created by cleavage fracturing (Figure 6c,d, Table 3, chemical analysis). The vermiculite had an intermediate composition between pure vermiculite and biotite, where K was not completely lost (Table 3). In the fine- to medium-grained sandstone from the outcrop sample KT2 of the Shakubetsu Formation, deformation bands (Figure 7) were barely seen as faint streaks in hand samples by naked eyes. In this sandstone, the matrix was entirely cemented by dolomite (Figure 7c). However, thin polished sections of this sample revealed that spaced deformation bands developed, which were defined by the alignment of detrital chlorite or biotite grains (phyllosilicate bands, Figure 7a). These deformation bands were oriented nearly parallel or slightly oblique to the bedding plane. Also, these phyllosilicate grains were again kinked due to the slipping along cleavages (Figure 7a). In one case, a conjugate set of deformation bands developed, where the dihedral angle between the two deformation bands is c. 60\({}^{\circ}\) (Figure 7b,c). The fact indicates that the conjugate deformation bands could have been formed by the \(\sigma_{1}\) oriented at a low angle to the bedding plane. Also, the conjugate phyllosilicate bands were accompanied by cataclasics of detrital grains (Figure 7b,c). It should also be noted that composite veins consisting of pyrite (Fe\({}_{1.0}\)S\({}_{1.9}\) from the SEM-EDS  
Figure 6: Microphotographs of deformation bands in outcrop sample KT1. See Figure 1b for the locality. (**a**) Phyllosilicate bands. Note that extended biotite grains aligned parallel to the bedding plane (the short dimension of the microphotograph). (**b**) Enlargement of the part enclosed by a white frame shown in (**a**). Note that the biotite detrital grains were extended and kinked due to slipping along the cleavages. (**a,b**) Under plane-polarized light. (**c,d**) SEM-BSE microphotographs of altered biotite grains. Note that kinking and creation of pore space were caused by cleavage fracturing in biotite grains, and peeled-off biotite selvages were curled in (**c**) (shown by an arrow). In (**c,d**), antibigenic smectite grew in the pore spaces. Measurements points for microchemical analysis with SEM-EDS are indicated by filled red circles. See Table 3 for the compositions. Bt, biotite; Vrm, vermiculite; KIn, kaolinite, Sme, smectite.