A marker tephra bed close to the Lower-Middle Pleistocene boundary: Distribution of the Ontake-Byakubi Tephra Bed in central Japan

Abstract

a b s t r a c tTephrochronology is an exceptionally important tool in the precise regional correlation of Early and Middle Pleistocene sedimentary strata in Japan. The present study reveals that the Yukawa tephra 5 (YUT5) derived from the Older Ontake volcano, the Nezumigawa (Nzg) and Mitamitajima (Mtj) tephras of the Ina Bazin, and the Byakubi-E tephra (Byk-E) of the Boso Peninsula are the same tephra on the basis of their lithofacies, bulk grain composition, mafic mineral composition, major element composition of hornblende, and stratigraphic relationships with the dated tephras. We propose to call the series of tephras correlated with Byk-E the Ontake-Byakubi Tephra Bed (On-Byk Tephra) following the naming convention in which the tephra name consists of the names of the source volcano and the type location. The Matuyama-Brunhes Chronozone boundary occurs just above Byk-E in the type section of the Kokumoto Formation in the Kazusa Group, which is a candidate Global Boundary Stratotype Section and Point (GSSP) for the lower boundary of the Middle Pleistocene Subseries. Therefore, On-Byk Tephra becomes a critically important marker tephra bed for the Early-Middle Pleistocene boundary in central Japan. The present study indicates that the major element composition of hornblende can be a useful tool for identification and correlation of strongly weathered tephra layers such as Nzg and Mtj in which all the volcanic glass shards have been altered.

A full sequence of the Matuyama–Brunhes geomagnetic reversal in the Chiba composite section, Central Japan

“…3; Suganuma et al 2018). The Byk-E tephra bed, originating from the Older Ontake volcano in Central Japan (Takeshita et al 2016), was ratified as the base of the Chibanian Stage/Age.…”

Calcareous nannofossil biostratigraphy of the Lower–Middle Pleistocene boundary of the GSSP, Chiba composite section in the Kokumoto Formation, Kazusa Group, central Japan, and implications for sea-surface environmental changes

“…The analyzed intervals, including the tephra Byk-E (Takeshita et al, 2016), are in the middle part of the Kokumoto Formation. The formation mainly contains massive mudstones with thin sandstones and occasional mudstone/sandstone interbeds.…”

Formal subdivision of the Quaternary System/Period: Past, present, and future

“…The primary magnetization carrier is magnetite (titanomagnetite). Takeshita et al (2015) have shown that the Byakubi-E tephra can be traced widely across central Japan and is derived from the Older Ontake volcano~250 km to the west (Takeshita et al (2015). It serves as a potential marker for the GSSP, and a useful guide for the MatuyamaeBrunhes boundary particularly between the Tabuchi (proposed location of GSSP), Yanagawa, and Kogusabata sections that comprise the Chiba composite section (Suganuma et al, 2015 supplementary data).…”