Abstract
A characteristic widespread tephra, the Sakai volcanic ash and its correlatives, is found in many major Pliocene sedimentary basins in central Japan. The Sakai volcanic ash and its correlatives consist of thin glass shards of bubble-wall type with very small amount of biotite phenocryst. The chemical composition of glass shards, rich in K2O and poor in CaO, Ba and Sr, differs markedly from other Pliocene tephra beds. Although this ash was given a local name in each sedimentary basin, namely the Sakai volcanic ash and Tarumi volcanic ash in the Tokai Group, the Ichibe volcanic ash in the Kobiwako Group, and the B22 volcanic ash in the Kakegawa Group, we rename this ash as the Sakai volcanic ash and its correlatives. The age of the Sakai volcanic ash and its correlatives is estimated at about 4.1 Ma based on the stratigraphical horizon situated at the upper part of N.19 zone by Blow (1969) and close to the upper boundary of the Cochiti Subchron in the Gilbert Chron. The source of this ash is thought to be situated to the west of the Kinki district due to the westward increasing of grain size.
This tephra will play an important roll in reconstructing the environmental change and tectonic evolution since the Early Pliocene in Japan.
