Pliocene widespread tephra of the Komoro Group in central Japan; correlation between the Znp-Ohta Tephra Bed and the U-1 pyroclastic flow deposit in the Okui Formation

Abstract

The Plio-Pleistocene Okui Formation in the Komoro Group in central Japan is composed of lake and fluvial deposits and contains many pyroclastic deposits. A thick pyroclastic flow deposit within the Okui Formation, namely the U-1 pyroclastic flow deposit, includes biotite mineral and is composed mainly of volcanic glasses and coarse pumice fragments (average maximum grain size: 10 cm). The U-1 pyroclastic flow deposit can be correlated to the Znp-Ohta Tephra Bed, which is the early Pliocene (ca. 4 Ma) widespread tephra in central Japan, by petrographic properties and stratigraphic position. This correlation enables precise correlation of the early Pliocene formations between the sea sides (the Pacific Ocean and Japan Sea) and the inland regions. The U-1 pyroclastic flow deposit is thickest among the pyroclastic flow deposits previously correlated to the Znp-Ohta Tephra Bed. This suggests that the source area of the Znp-Ohta Tephra Bed is located near the distribution area of the Okui Formation.