Abstract
Stratigraphy of pyroclastic deposits around Akan and Kutcharo calderas in eastern Hokkaido was investigated in order to reconstruct the eruptive history of the Akan volcano, and to clarify the stratigraphic relationships among pyroclastic deposits of Akan and other volcanoes. In addition to the geological data, source volcano of each deposit was identified using petrological features of juvenile materials, such as their phenocryst assemblages, whole-rock and glass chemistry. On the basis of the presence of significant time intervals, indicated by paleosol and erosional gap, we recognized at least 40 eruptive units of pyroclastic deposits (Akan pyroclastic deposits). Many of them are successions of pyroclastic fall deposits followed by pyroclastic flow deposits. Based on petrological features of juvenile materials of each unit, we grouped these units into 17 eruptive groups (Ak1 to Ak17 in descending order). Each group is composed of a single or sequential series of eruptive units. Whole-rock chemistry of juvenile materials is different from one group to another. This suggests that activities of each eruptive group were derived from distinct magma system.
We found several exotic pyroclastic flow deposits and distal air-fall ash layers within the Akan pyroclastic deposits. Pyroclastic flow deposits from adjacent Kutcharo caldera are interbedded between Ak2 and Ak3. One of them was dated to be 0.34 Ma. Distal rhyolitic ash layers from central Hokkaido (1.3−1.46 Ma) are recognized within Ak14. These dates suggest that the large-scale explosive volcanism at Akan volcano started in the Early Pleistocene and repeated many times for more than one million years. During the activity, the magma system was repeatedly renewed to produce eruptive group characteristics. This complex and long-term eruptive history of Akan volcano is probably reflected in the rectangle shape of Akan caldera. The shape of the possible composite caldera is different from the circular shapes of many other calderas.
