Depositional process of the Byk‐E tephra bed in the Chiba section, central Japan: A marker bed defining the Global Boundary Stratotype Section and Point for the Chibanian Stage

Abstract

AbstractThe global boundary stratotype section and point (GSSP) for the Chibanian Stage and Middle Pleistocene Subseries is present at the base of the Byk‐E tephra bed in the Chiba section, located in the central Boso Peninsula, Japan. The Byk‐E tephra bed, which erupted from the older Ontake Volcano, is a conspicuous marker bed in the Chiba section; therefore, it provides a clear local and regional marker for the GSSP. However, the sedimentological characteristics of the Byk‐E tephra bed have not been well documented. Here, we provide a detailed description and an isopach map of the Byk‐E tephra bed based on the lithofacies, grain‐size compositions, and grain orientations in the Chiba section and other localities in the Boso Peninsula. The lithofacies and grain size compositions of the tephra bed show that the lower part of the bed show no evidence of flow‐induced deposition. These results suggest that the Byk‐E tephra bed is most likely deposited as fallout deposits under a weak bottom current that could not direct the heavy mineral grains. Therefore, the Byk‐E tephra bed is not to erode the underlying bed during the deposition, indicating that there is no stratigraphic discontinuity. The isopach map drawn from the observed thickness of the fallout unit, show that the distribution of the Byk‐E tephra extends more than 260 km but a limited azimuthal range.