Abstract
We investigated the correlation between pyroclastic materials in terrigenous marine deposits of the Kushiro region and pyroclastic deposits in the Akan volcanic area, eastern Hokkaido, with respect to Quaternary stratigraphy, glass chemistry, contents and assemblages of phenocrysts, and age determinations using tephro-chronology. A high-resolution Pleistocene tephro-stratigraphy of the Akan volcano was previously established for the Ak1–Ak17 sequence (youngest–oldest). The Akan eruptive deposits are intercalated with time-marker tephras derived from the adjacent Kutcharo volcano (KpI–KpVIII, youngest to oldest), central Hokkaido, and from unknown sources. In this study, pyroclastic materials in marine deposits in the Quaternary Otanoshike Formation, Kushiro region, were sampled at four type localities.
The Otanoshike Formation, covered by two pyroclastic flow deposits, contains two volcanic ashes, one in the lower part and one in the upper part of the formation. The lower and upper ashes correlate with the LowK-1 (source unknown) and Ak5 tephras, respectively. The overlying lower and upper pyroclastic flow deposits correlate with the KpVI and KpIV, respectively. The ages of the LowK-1 and KpIV tephras were estimated at ca. 0.8 and 0.1 Ma, respectively. These ages indicate that deposition of the Otanoshike Formation started by at least 0.8 Ma and finished before 0.1 Ma.
The Kushiro Formation, composed of the lower Takkobu and the upper Toro members, contains many reworked tephras, all originating from the Ak13–Ak17 tephras, implying that the Kushiro Formation formed before the Ak12 eruption. The Ak14 and Ak10 eruption ages are estimated at ca. 1.5 and 1.0 Ma, respectively, suggesting that deposition of the Kushiro Formation spanned the 1.5–1.0 Ma time period.
