北海道洞爺湖周辺の第四紀火砕岩及び火山岩のTL年代

抄録

Thermoluminescence (TL) dating was made for volcanic rocks and pyroclastic deposits around Toya Caldera, Southwest Hokkaido. TL ages of Nakajima lava dome in the caldera are 0.040, 0.041 and 0.045 Ma, and those of Toya Pyroclastic Flow Deposit are 0.103-0.134 Ma (average: 0.119 Ma for 6 samples). Both data coincide well with the ages which were estimated indirectly from ¹⁴C dating of the related pyroclastic deposits and a previously reported fissiontrack age, respectively. TL ages of the Takinoue Pumice Flow Deposit (0.90 Ma) and the Sobetsu Pumice Flow Deposit (1.59 Ma) indicate them to be middle and early Pleistocene, respectively, although they were vaguely considered as Pliocene or Pleistocene in age. A TL age for Miocene Osarugawa Formation showed only 2.57 Ma, which is interpreted as an age started from the time when the temperature of the formation decreased below 40-50°C. In case of a low density pumice sample, a reasonable age was obtained in terms of weight correction.