Characterization and correlation of the Hegawa‐Kasamori 5 tephra, a widespread tephra aged c. 450 ka associated with large‐scale pyroclastic flows from southern Kyushu, SW Japan

Abstract

ABSTRACTA Middle Pleistocene widespread tephra, defined here as Hegawa‐Kasamori 5 tephra (Hgw‐Ks5), has been newly recognized over a broad area of Japan. Large‐scale pyroclastic flow deposits associated with co‐ignimbrite ash fall deposits (CAFDs) of Hgw‐Ks5 have been identified in the proximal southern Kyushu area, south‐west Japan. Hgw‐Ks5 possibly originated from the Aira caldera in southern Kyushu, and it is widely spread and intercalated with deposits of the Kasamori Formation, Honshu Island, more than 1000 km away from the source. In the north‐west area of the Aira Caldera, the tephra is sparsely distributed in the form of non‐welded ignimbrites, and is exposed stratigraphically above the well‐known Kobayashi‐Kasamori tephra. Hgw‐Ks5 is characterized through petrographic features, major element geochemistry of glass shards, and refractive indices of orthopyroxene. The results of previous stratigraphic isotope studies indicate that the eruptive age of Hgw‐Ks5 is 434–458 ka (Marine Isotope Stage 12). Assuming that the CAFDs originating from the Aira Caldera are distributed concentrically, the apparent volume of Hgw‐Ks5, estimated from the area of distribution and CAFD thickness, is ~100 km3. Therefore, a volcanic explosivity index of 7 is assigned to the Hgw‐Ks5 eruption.