Some Considerations on the Stratigraphic Relationship between the Late Cenozoic Katsuura and Kurotaki Formations in the Katsuura District, Boso Peninsula

Abstract

1) This research was carried out, in order to clarify the stratigraphic relationship between the Katsuura and the Kurotaki Formations, particular attention being given to the tephra key bed known in the Katsuura district, which is named Kr in this article. The tephra key bed, outcropping in the area around Kasugadai and the Katsuura Bridge, in Katsuura city, was dealt as typical Kr.
2) In the eastern part of the area, the strike of Kr runs NNE-SSW, but ENEWSW in the western part. Since Kr is cut by several normal faults, its distribution is slided horizontally and vertically.
3) The tephra key bed (Kr) consists of three members, KRl, KRm and KRu, in ascending order. Among them, KRm, the middle one, is divided into five beds, m0, m1, m2, m3 and m4.
4) Kr extends from the east, where Katsuura Formation is distributed, to the west where the Kurotaki Formation is distributed, through the Ishikiri and the Aiyama valleys. In the Aiyama valley, Kr is situated stratigraphically about 8m above the plane of the Kurotaki unconformity, but Kr disappears at the cliffs cut by the city-road about 0.6km to the west of the Aiyama valley.
5) Each member of Kr becomes thinner in the western part than in the eastern part. At Matsube its total thickness is about 12m, but becomes about 9m in the Ishikiri valley, and about 4.5m in the Aiyama valley.
6) In the western part of Kr, its rock characteristic changes distinctly and becomes the facies of the coarsegrained tuffaceous sandstone with the pumice and the scoria.
7) If Kr is considered as a standard horizon, the Kurotaki Formation, in the Kurogahana area in the most-eastern part of the district, could be regarded to be the lowest horizon in its distribution, and also as the oldest. On the other hand, the Formation distributed in the most-western part of this district is the upper horizon and also younger than that of the most-eastern part of the district.
8) It is, therefore, concluded that the Kurotaki Formation was deposited at different times with same facies, and also that the Katsuura and the Kurotaki Formations were deposited at the same time with different facies. It is remarkable that the Kurotaki Formation in the Kurogahana area lies under the Katsuura Formation as its basement.