The Kakegawa District is the type area of the Japanese Neogene. The Neogene/Quaternary boundary has been known at the top of Flysch type alternation of Horinouchi facies belonging to lower and middle Kakegawa Group. The upper part of Kakegawa Group consists of sandy shallow water deposits of the Soga Formation in the north and massive mud of the Hijikata Formation in the south area. The Ogasa Group unconformably overlying the Kakegawa Group is composed mainly of the gravels of the Ooigawa River Basin. In the north part, it intercalates many mud beds of marsh, lagoon and intertidal zone and several beach pebble beds.
The normal paleomagnetic polarities of the top of the Hijikata mud in the south are correlated to the Jaramillo event. The lower part of Ogasa Group has reversed polarities and contains Metasequoia cones. The normal polarities of the upper part of Ogasa Group are correlated to the Brunhes Normal. The uppermost mud bed of Ogasa Group, the Okazaki mud yields warm-water and inlet shell fossils, such as Dendostraea paulucciae and Anadara granosa. This bed may be correlated to Ma 8, the uppermost warm-marine clay of Osaka Group or to Ma 10 in the Higher Terrace sediments.
