Fission-track age of the volcanic ash layer in the Bisan-seto Group around the Bannosu region of the eastern parts of the Seto Inland Sea, Japan

Abstract

Fission-track dating of zircon and volcanic ash analysis were carried out for Bannosu volcanic ash in the Bisan-seto Group around the Bannosu region in the northeast Shikoku. The first reliable radiometric age, 0.63 ± 0.14 Ma(1σ) , was obtained for the volcanic ash layer. On the other hand, the refractive indexes of the volcanic glasses show the 1.5004-1.5071(mode: 1.504-1.506) . On the basis of these values and other petrographical data, this volcanic ash layer is correlated to the Kb-Ks volcanic ash. These results are important to discuss the sedimentary environment of the Seto Inland Sea in Pleiocene to Holocene.