The Sotoyama Plateau on the mid-western Kitakami Massif, Northeast Japan, displays a low-relief surface with an altitude of 650-1000 m. This topography is considered to have been formed by periglacial phenomena. We discovered the 80-cm-thick pyroclastic fall deposit, named Yabukawa Tephra in this study, from the Quaternary valley floor deposits along the Sotoyama River on the plateau. This tephra is subdivided into four subunits on the basis of their lithofacies. The tephra contains

more »

... ically large amounts of vesiculated pumice and minor lithic fragments: andesite, dacite, rhyolite, tuffaceous mudstone, tonalite, chert, etc. It also contains grains of beta-quartz, feldspar, hornblende, orthopyroxene, ilmenite, biotite, etc. A geochemical composition of volcanic glasses from the tephra shows relatively high SiO 2 and K 2 O, and low CaO, MgO and TiO 2 . Their refraction indexes are 1.495-1.498. The Yabukawa Tephra is thought to deposit in the late Chibanian on the basis of the 0.24 ± 0.04 Ma fission-track age from zircon in its pumice. Furthermore, the tephra is possibly correlated with the Odai White Volcanic Ash, which occurs in the eastern foot of Mt. Iwate, based on its petrography, refraction indexes of volcanic glasses and the zircon age.