支笏およびクッタラ火山のテフロクロノロジー

抄録

The surrounding area of the Shikotsu and the Kuttara volcanoes, and Ishikari-Lowland, in the southwestern Hokkaido, are covered with thick pyroclastic deposits of the late Pleistocene. These pyroclastic deposits have been subdivided into units according to facies changes. consequently each unit does not necessarily represented a single eruption. In the present paper, the products of one eruption (tephra formations) are separated from one another by recognition of intervening “volcanic ash soils” which are considered to be the most suitable for indicating quiet period, owing to its low depositional rate and generality.
In the Ishikari-Lowland, the previously defined two groups of tephra formations (En-c and Spfl · Spfa 1; Spfa 7, 8, 9 and 10) are reinterpreted to represent the products of two eruptions, because the volcanic ash soils are absent among them. And three tephra formations are newly recognized. Twenty two tephra formations constitute the late Pleistocene pyroclastic deposits in the Ishikari-Lowland.
The pyroclastic deposits distributed around the Kuttara volcano are subdivided into twelve tephra formations and one scoria fall group. The tephra formations derived from the Kuttara volcano are as follows in ascending order : Kt-8, 7, 6, 5, 4, Kt-Hy, Kt-3, Kt-Tk scoria fall group, Kt-2, Kt-1. The Kt-8 …-1 are mainly composed of plinian pumice fall deposits and pumice flow deposits generated by large and moderate scale explosive eruptions.
The correlation of the tephra formations in the surrounding area of the Kuttara volcano and the Ishikari-Lowland is attempted with several petrographic parameters and stratigraphic position. The following tephra formations are correlated each other : Kt-8 and Aafa 1, Kt-5 and Mpfa 2b, Kt-4 and Mpfa 2a, Kt-3 and Spfa 4, Kt-Tk and Spfa 3, Kt-1 and Spfa 2. In addition, it is proved that the following tephra formations spread to the Ishikari-Lowland : Kt-7, Kt-Hy, Nj-Os.
On the Shikotsu volcano, three explosive eruptions preceded a large scale caldera forming eruption (Spfl · Spfa 1 : Ca. 32 ka), and after a short repose time, present post caldera activity started. The Kuttara Volcano was the site of seven explosive eruptions during the period from 70-40 ka ; then the stratovolcano was formed by erupting Kt-Tk scoria fall group, and the summit caldera was formed by the last two explosive eruption (Kt-1 and 2).