[Abstract]

The drilling core (501.9 m in depth) has been obtained from Okogashima Island located in southern margin of the Aira caldera at northern end of Kagoshima Bay in southern Kyushu. The core reveals detailed eruption history prior to the Aira pyroclastic eruptions about 25,000 years ago by descriptions of lithofacies and fissiontrack dating. The three types of lithofacies are identiÉÖd in the drilling core. They are (1) 2-8 m in depth: pyroxene andesite; (2) 8-253.2 m in depth: the Okogashima rhyolite lava flow; (3) 253.2-501.9 m in depth: pyroclastic materials. This study focuses mainly on pyroclastic materials, which have not been described yet. The pyroclastic materials in lower levels are composed mainly of tuffaceous sand with cross lamination, containing shell fossils. They also contain pumice-concentration zone and welded tuff, indicating explosive eruption. The pumice-concentration zone occurs in 326-340 m (unit A), 354-360 m (unit B), 425-427 m (unit D), 449-450 m (unit E), and 478-497 m (unit F), while the welded tuff occurs in 360-380 m (unit C). The fission-track ages of around 0.4 Ma from the units B and C indicate that the deposits were erupted prior to the Aira pyroclastic eruptions. Therefore, this drilling core shows that at least sixth pyroclastic eruptions are identified in southern margin of the caldera prior to the Aira pyroclastic eruptions.

Key words:

Aira caldera, drilling core, fission track, welded tuff, Yoshino pyroclastic flow

e-mail:

furukawa@gaia.h.kyoto-u.ac.j