掛川地域の第四紀層  [in Japanese] Quaternary System of the Kakegawa Area, Central Japan  [in Japanese]

The Kakegawa area on the Pacific coast of central Japan is well known as one of the type areas of the Japanese Cenozoic. The Pleistocene strata in the area are more than 500 m thick. They are composed of marine and freshwater sediments, and are divided into three units, the upper part of the Kakegawa Group 5 the uppermost part of the Kakegawa Group named the Soga Formation, and the Ogasa Group. The upper part of the Kakegawa Group consists of the Hijikata Silt and the Aburayama Sand. The Soga Formation lies on the upper part of the Kakegawa Group with a local unconformity in the north of the area. It consists of conglomerate, sand and silty sand in the north west and of massive silt in the southeast. Many acidic tuff layers are intercalated in the Kakegawa Group and they are useful key markers to chronostratigraphical subdivision. The Kakegawa Group yields abundant marine fossils including molluses and foraminifers. The Ogasa Group, unconformably overlying the Kakegawa Group, is composed mainly of gravels of terrestrial origin. It intercalates many mud beds which were deposited in marshes, lagoons and inner bays. Some plants and molluses are included in these mud beds. Planktonicforaminiferal biostratigraphy of the Kakegawa Group is synthesized by IBARAKI (1986a, b). 4 datum planes are recognized in horizons near the Plio-Pleistocene boundary, ite. Pulleniatina spp. DS datum, Globorotalia truncatulinoides Base datum, Neogloboquadrina asanoi top datum and Pulleniatina spp. Second SD datum in ascending of der. Magnetostratigraphy of the area were summarized by NAKAGAWA et al. (1982) on the basis of the results of YOSHIDA and NIITSUMA (1976), and ISHIDA et al. (1980). The patterns of the spatial and temporal distributions of molluscan assemblages in the Kakegawa Group are briefly shown (Fig. 5). Molluscan and floral fossils of the Ogasa Group are described as keys to the age and paleoenvironment. The chronostratigraphy of Pliocene and Pleistocene strata in the Kakegawa area is established on the basis of the results of biostratigraphy and magnetostratigraphy (Fig. 6). The Makinohara and Mikata-gahara Gravel Beds, and Furuya Mud which occupy the uppermost part of the Middle pleistocene, are shortly noted and their distribution is represented in the geologic sketch map.