Abstract
Serpentinite bodies are distributed along fault boundaries between the Sanchu Cretaceous and the rocks of the Southern Chichibu Belt in the northwestern Kanto Mountains, central Japan. It has been thought that the relationship of the Sanchu Cretaceous and the serpentinite is a fault contact, but we found an outcrop where the Shiroi Formation (brackish sediments) of the Cretaceous covers the serpentinite unconformably. The contact surface is undulatory in geometry and is oblique to the bedding plane of the basal Shiroi Formation. Serpentinite clasts are included in the basal part of the Shiroi Formation. The microscopic texture of serpentinite clasts consists mostly of antigorites and differs from the textures of underlying serpentinite.
It is clear that the serpentinite occurs in two different manners, that is, a constituent of the fault zone and a basement rock of the Sanchu Cretaceous. Almost serpentinite clasts were derived from antigorite-rich serpentinite bodies nearby. We can conclude that the serpentinite body along the Otchizawa River was protruded on the brackish area immediately before the initial sedimentation of the Shiroi Formation (Hauterivian).
