The age and stratigraphic horizon of the Omachi APm tephra beds, Middle Pleistocene time-markers in central Japan

Abstract

Five distinctive tephra beds (A1Pm~A5Pm), carrying abundant biotite and quartz crystals, are widely distributed in central Japan. They are here collectively called Omachi APms or APms in short, and are petrographically identified by high refractive indices of hornblende and orthopyroxene. One bed of the APms is found as far as 239 km from the presumed source, Momisawadake in the Hida Mountains, where it is 4 cm thick and composed mainly of silt-sized glass. No proximal deposit made up of coarse pumice fragments is found owing to unfavorable preservation of the mountainous landscapes; however, the eruption style is supposed to be of plinian type based on analyses of grain size along the dispersal axis. Assuming that the accumulation rate of loess soil has been constant at each locality, the age of APms is estimated 0.24~0.33 Ma by the overlying soil thickness. -from English summary