Tephrostratigraphy of the Shinjo Basin, Yamagata Prefecture, Northeast Japan, in the Latter Half of the Middle Pleistocene

Abstract

Tephra layers in the latter half of the Middle Pleistocene of the Shinjo Basin are distinguished by differences in mineral assemblage, shape of volcanic glass shards, and refractive indices of volcanic materials.
Nishiyama tephra (Nsy) is nitric with small amounts of orthopyroxene, amphibole, and quartz. The refractive index of volcanic glass in this tephra shows unimodal distribution but the refractive indices of orthopyroxene and amphibole vary widely. Nsy is also found in the Kiyotaki Hills, on the eastern side of the Ou Range.
Torigoe tephra (Trg) containing volcanic glass, orthopyroxene, clinopyroxene, amphibole, and quartz is correlated with Onikobe4kezuki tephra (0-Ik). The refractive index of volcanic glass in this tephra shows vertical variation but that of orthopyroxene is relatively constant.
Nimaibashi tephra (Nmb) is vitric with small amounts of orthopyroxene and quartz. The refractive index of glass in this tephra shows bimodal distribution and that of orthopyroxene varies widely.
Izumikawa tephra (Izk) is vitric with small amounts of orthopyroxene and quartz. The refractive index of volcanic glass in this tephra shows vertical variation and that of orthopyroxene varies widely.
Emakawa tephra (Emk) contains volcanic glass, amphibole, orthopyroxene, and quartz. The refractive indices of volcanic glass and orthopyroxene show unimodal distribution but the refractive index of amphibole shows trimodal distribution. This tephra is correlated with Magarizaka tephra (MgA).
Ushikuguri tephra (Usk) contains volcanic glass, orthopyroxene, and quartz with small amounts of amphibole. The eruptive age of Usk is estimated to be 200-230 ka as long as the accumulation rate of loess above Usk is constant between 11 to 14 mm/kyr. Emk and 0-Ik are also estimated to have eruption ages of (200-230) -260 ka and ca. 300 ka, respectively.
The Yamaya formation, which represents the top of the Shinjo Basin sediments, includes 0-Ik tephra in the middle part. Therefore the formation emerged after 0-Ik eruption and the age of emergence is estimated to be ca. 300 ka.