2019 Volume 73 Issue 3 Pages 179-184
The characteristic blocks of massive conglomerate including granitic cobbles and pebbles were discovered from the Yokozemachi Group of uppermost Neogene in the Chichibu Basin. These blocks are seen as pebble to boulder in the Neogene unconsolidated conglomerate, and are sub-angular to sub-round gravel. The gravel composition of massive conglomerates is mostly sandstone and mudstone, containing granitic rock, granite porphyry, andesite, and hornfels, while no sandstone or mudstone of the Neogene origin. Its sorting is bad, and its matrix is consolidated. Biotite in the matrix of massive conglomerates accounts for about 7.3% in mode composition. Such features are very similar to conglomerate of the Cretaceous Sanyama Formation in the Sanchu Graben. Therefore, it is deemed that during the sedimentation of the Yokozemachi Group, the Cretaceous system of the Sanchu Graben was exposed to the upper part of the Chichibu Belt adjacent to the east, from which massive conglomerates including the granite and other rocks were supplied.
