Abstract
Six distal rhyolitic ash layers (HR-1∼HR-6 in descending order) are interbedded in the pile of large-scale pyroclastic flow and pumice fall deposits derived from Akan volcano, eastern Hokkaido, during Pleistocene. These layers characteristically contain hydrous minerals such as hornblende and biotite, which are not common in the rocks of Quaternary volcanoes in eastern Hokkaido. Based on mineral assemblage, glass chemistry, and stratigraphy, these can be correlated with the early to middle Pleistocene large-scale pyroclastic flow deposits (pfld) distributed in central Hokkaido. The HR-5 and -6 layers have been already correlated with the Tokachi pfld (1.3∼1.46 Ma) in the central Hokkaido. In addition, we have newly correlated these ash layers to pfld in the central Hokkaido, as follows: HR-4 to the Tokachimitsumata pfld (1 Ma), HR-2 to the Kamishikaribetsu pfld and, HR-1 to the Kamiasahigaoka pfld, respectively. K-Ar ages for plagioclase in pumice of Kamiasahigaoka pfld is determined to be 0.51±0.14 Ma. These dates suggest that the large-scale felsic explosive volcanism had continued in both of central and eastern Hokkaido for at least eight hundred thousand years long during the early to middle Pleistocene. Especially, there is no evidence for dormancy, such as paleosol and erosional gap, between Ak14 and intercalating HR-5, indicating that large explosive eruptions simultaneously occurred in central and eastern Hokkaido. HR-1∼HR-6 could be Quaternary good time markers not only in eastern Hokkaido but also in Pacific Ocean and Kurile Islands because these tephras are more than 50 cm in thickness around Akan volcano.
