Outline of geology

The Boso Peninsula is situated in the southern part of the Kanto region in central Honshu, between two major tectonic provinces of the Japanese Islands: northeastern Honshu, extending to the north, and southwestern Honshu, extending to the west. Thick marine sediments accumulated in a depositional basin that appeared in the middle Pliocene in the southern part of the Kant region; this Plio–Pleistocene section is well exposed in the Boso Peninsula.

The Tertiary and Pleistocene deposits in the Boso Peninsula consist of the Mineoka, Hota, Miura, Kazusa, and Shimosa groups, in stratigraphic order, together with younger terrace formations and volcanic-ash layers. Each of the groups is unconformable with the others. Planktonic foraminifera indicate that the Miocene–Pliocene transition is in the uppermost part of the Amatsu Formation.

After deposition of the Anno Formation of the uppermost Miura Group, the area was uplifted, and a new basin, in which the Kazusa Group was deposited, appeared to the north of the emerged area. In the central-to-eastern parts of the peninsula, the Kazusa Group comprises the Kurotaki, Katsuura, Namihana, Ohara, Kiwada, Otadai, Umegase, Kokumoto, Kakinokidai, Chonan, Mandano, and Kasamori formations, in ascending order. Those formations consist of sandstone and siltstone, with pyroclastic intercalations, and the classification of the formations is based on the predominant lithology in the interbedded rocks.

The pyroclastic intercalations of the Kazusa Group have been carefully traced in the Boso, Choshi, and Miura peninsulas, where they are used as key beds. Lateral changes in lithology and local stratigraphy have been well established (Mitsunashi and Yazaki, 1958, 1961; Mitsunashi et al., 1959, 1961, 1976a,b, 1979; Yazaki and Mitsunashi, 1962; Mitsunashi and Yazaki, 1968; Ishiwada et al., 1971).