Identification of a pair of co-ignimbrite ash and underlying distal plinian ash in the Early Pleistocene widespread tephra in Japan

The Early Pleistocene Azuki and the Ku6C tephras in Japan have been correlated with a compositionally zoned ignimbrite, the Imaichi pyroclastic-flow deposit, using ratios of accurately determined refractive indices of volcanic glass shards and phenocrysts and paleomagnetic directions. Thin, fine-grained, white ash layers at the base of the Azuki and the Ku6C tephras have been correlated with the Imaichi plinian-fall deposit, which underlies the Imaichi pyroclastic-flow deposit near the source Shishimuta caldera on central Kyushu. This indicates that the Azuki and the Ku6C tephras comprise a pair of compositionally zoned co-ignimbrite ash and underlying distal plinian ash. Thus, the initial sequence of physical and chemical changes at the source of the pyroclastic eruption has been retained in the distal widespread tephras, even though the ashes were blown downwind by westerlies as much as 900 km. Identification of a distal plinian ash at the base of a co-ignimbrite ash ensures the stratigraphic correlation of the distal tephra with the source ignimbrite.