Abstract
The late Pleistocene pyroclastic fall deposits erupted from Yotei Volcano, a typical strato-volcano in the southwestern Hokkaido, Japan, are found extensively from the eastern foot of the volcano to the Sapporo-Tomakomai Lowland. The three characteristic marker tephras were extracted and their isopachous lines were drawn (Figs. 2 and 3).
They are sandwiched between the two well-known marker tephras, the Shikotsu pumice flow deposits (Spfl) and the Eniwa-a pumice fall deposits (En-a), and were named Yo. Ps-3, Yo. Ps-2 and Yo. Ps-1 in ascending order. Previously the Yo. Ps-3 and Yo. Ps-1 have been mistakenly assigned to the tephras of Eniwa Volcano.
Yo. Ps-3 and Yo. Ps-2 are augite-hypersthene andestic in composition with the 14C dates of about 25, 000-30, 000y.B.P. Yo. Ps-1 which was deposited about 15, 000 14C y.B.P. is hornblende-augite-hypersthene andestic with apatite microlite.
The pumice fall deposits rich in hornblende overlain by Yo. Ps-3 and the tephras underlain by Yo. Ps-1 which are assigned to the Yotei are observed in the eastern foot of the volcano. Further investigations on these tephras and other volcanic ejecta will clarify the eruptive history and the growth process of the conical volcano.
