Abstract
This paper presents new evidence regarding the depositional age of mudstone in the Shiraki Group within the Northern Belt of the Chichibu Composite Belt, Shima Peninsula. We address three issues: 1) the stratigraphic relationship between the Shiraki Group and the Shirataki Group (within the Kurosegawa Belt); 2) the position of the boundary between the Northern and Kurosegawa belts; and 3) the tectonostratigraphic correlation with the standard scheme for the entire Northern Belt in the Japanese Islands.
The Shiraki Group is the southeasternmost stratigraphic unit of the accretionary complexes within the Northern Belt to the north of the Gokasho–Arashima Tectonic Line. Radiolarians extracted from mudstone are characterized by Eoxitus elongatus, Eucyrtidiellum unumaense, Hsuum crassum, Hsm. maxwelli, Minutusolla aff. latusicostata, Mnt. nishimurae, Mnt. cf. michelei, Praewilliriedellum cf. convexum and Striatojaponocapsa synconexa. This fauna indicates that the mudstone of the Shiraki Group is middle Middle Jurassic (Bajocian–early Bathonian) in age.
The Shirataki Group was previously assigned to the Kurosegawa Belt to the southwest of the Shiraki Group. It shares common lithofacies, geological structures and radiolarian ages with the Shiraki Group, suggesting both groups can be regarded as a single stratigraphic unit. Integrating these units leads to a southward shift of the boundary between the Northern and Kurosegawa belts.
The scheme of standard stratigraphic units applicable throughout the entire area of the Northern Belt in the Japanese Islands encompasses five units. The lithofacies and depositional ages of the Shiraki Group show striking similarity to those of the Kamiyoshida Unit. Thus, the Shiraki Group is probably correlated with the Kamiyoshida Unit.
