Abstract
The Southern Chichibu Belt in the western Chichibu area is named the Nakatsugawa Group and has been proved to be composed of Jurassic accretionary complexes. The Nakatsugawa Group has been divided into the Ogamata, Ryokami, Nogurizawa and Ryokami-yama Chert Units in structurally ascending order. This study reports the geology of the Ryokami Unit around Shigehagi-yama in the northeast part of Otaki, Chichibu City, Saitama Prefecture, Kanto, Japan. The major lithofacies of the Ryokami Unit in the study area is sandstone and mixed or broken facies of sandstone and mudstone. Small amounts of limestone, chert and mixed facies composed of muddy matrix and sandstone, limestone, chert and greenstone blocks are also present. The thickness of the limestone bodies in the study area reach approximately 300 m, which is greater than the thickness of previously reported limestone bodies in the Ryokami Unit. The geological structure of the study area is characterised by NE-SW strikes and steep northward dips, judging from the orientation of bedding planes, the orientation of cleavage planes and the distribution of lithofacies. In the west of Shigehagi-yama, a NNE-SSW striking, steeply eastward dipping fault cuts the general geological structure. The limestone body that is distributed from the peak of Shigehagi-yama to the settlement of Ochiai is accompanied by mixed facies composed exclusively of basaltic matrix and limestone blocks. The mixed facies composed exclusively of basaltic matrix and limestone blocks was probably formed by the tectonic mixing of the boundary between basaltic rocks and limestones of a pelagic seamount.
