Abstract
The Miyada Formation was originally described by Aoki (1925) to include the marine strata distributed on the Miyada Plateau of the southern part of the Miura Peninsula. This Formation unconformably overlies the Miocene Miura Group and is overlain by upper Pleistocene strata. The formation, which consists mainly of sand, is well known to yield a large number of molluscan fossils. Okumura et al. (1979) reported 253 species of molluscan fossils from the Miyada Formation.
Recently, the authors determined the geological ages of the Miyada Formation by the Electron Spin Resonance-method (ESR). These ages show 513, 000±63, 000 years (lower part of the Miyada Formation, using the shell of Glycymeris yessoensis) and 325, 000±40, 000 years (Upper part of the Miyada Formation, using the shell of Felaniella usta). The authors considered that these ages of the Miyada Formation are not contradictory those identified by the former studies, e. g. 300, 000-370, 000 years of age (Ohmura, 1991) and 400, 000 years of age (Yamada et al., 1983). The Miyada Formation is possibly correlative with the Zizodo Formation and the Yabu Formations of the Simosa Group developed in the Boso Peninsula. This correlation was based on the ESR and Fission Track methods and a comparison of the characteristics of the molluscan fauna in the two areas.
