北海道中央部，美瑛～上川地域に分布する十勝カルデラ周辺の火砕流堆積物のFT年代及びU-Pb年代

抄録

Zircon fission-track (FT) and uranium-lead (U-Pb) dating were carried out to determine the ages of the Biei, Tokachi, and Sounkyo pyroclastic flow deposits in the Biei and Kamikawa areas of central Hokkaido, northern Japan. We collected pumiceous tuff samples of the Biei pyroclastic flow deposits from two sites in the middle and lower reaches of the Biei River, and Tokachi pyroclastic flow deposits from one site to the west of Tokachi caldera. A sample of welded tuff from the Sounkyo pyroclastic flow deposits was obtained from one site in the lower reaches of the Antaroma River. The FT ages of the Biei pyroclastic flow deposits are 0.81±0.08 Ma and 0.72±0.08 Ma, identical to each other within 1σ error. However, they differ from an age of 1.91±0.06 Ma reported previously from the upper reaches of the Biei River. Based on the present data and previous results on the ages and petrographical characteristics of the deposits, they can be divided at least two geological units with different eruption ages. A FT age of 0.058±0.018 Ma (1σ) was obtained from the Sounkyo pyroclastic flow deposits. On the basis of previous studies concerning the distribution and petrographical characteristics of the deposits, this age was obtained from Hb-type pyroclastic flow deposit among the Hb- and Py-type flows of the Sounkyo pyroclastic flow deposits. The Tokachi pyroclastic flow deposits yielded a U-Pb age of 1.24±0.02 Ma (2σ), which falls within the wide range of ages reported in previous studies. Because the Tokachi pyroclastic flow deposits have a wide distribution and a wide range of ages, they can be divided into several geological units with different eruption ages, as with the Biei pyroclastic flow deposits.