The Plio-Pleistocene Osaka Group was deposited in the Second Setouchi depression and its thickness attains up to 1,000m.
Osaka Group has twelve marine clay beds in the upper part (Ma-1, Ma-0, Mal-10) and are intergalated by about fifty layers of thin volcanic ash as marker-tephra.
The author studied major element chemical composition of glass of these marker-tephras using a microprobe analyzer and described the successful characterization for correlation to be made for these marker-tephras.