Fission track ages of pyroclastic flows in the pliocene Ashoro Formation and the Plio-Pleistocene Ikeda Formation developed in eastern Hokkaido, Japan

Fission track ages of the three pyroclastic flow deposits developed in the Tokachi Province, eastern Hokkaido, have been determined by means of grain by grain method of zircon extracted from these deposits. The following results have been obtained; 3.7 +- 0.1 m.y. for the Inashibetsu pyroclastic flow deposits of the Ashoro Formation; 2.8 +- 0.2 m.y. for the Sarubetsu pyroclastic flow deposits; and 2.0 +- 0.1 m.y. for the Chiyoda pyroclastic flow deposits, both the latter two are of the Ikeda Formation. Pliocene-Pleistocene boundary in this Province must therefore lie somewhere which may be slightly above the horizon of Chiyoda pyroclastic flow deposits of the Ikeda Formation. (author)