The Pumice-fall Deposits of the Late Pleistocene in the Tokachi Plain, Hokkaido

Abstract

In the Tokachi Plain of the south-eastern Hokkaido, a number of the pumice-fall and ash-fall deposits ranging from the late Pleistocene to Holocene are widely developed. As shown in Fig. 1, most of these deposits were accumulated on the southeastern or eastern side of the volcanoes.
Some of these pumice-fall deposits of the late Pleistocene, that is, Orange-colored pumice-fall 3, 2 and 1 (Op-3, Op-2 and Op-1), Shikaribetsu pumice-fall 2 and 1 (Sipfa-2 and Sipfa-1), and Eniwa-a pumice-fall deposits (E-a) were found in this plain by the studies of the Tokachi Research Group, since 1962. This paper is a summary of these studies (Figs. 2, 3, 4, 5, 6, 7 and Table 1).
In the last glacial period (W I-II), the orange pumice-fall 3, 2 and 1, Shikotsu pumice-fall 2 (Spfa-2) and pumice-fall 1 (Spfa-1) erupted from the Shikotsu volcano are mainly extending to the southern part of Tokachi Plain and across the Hidaka Mountains (Figs. 1 and 3). The Op-1 pumice-falls serupted as the earlier stage ejecta of the Shikotsu volcano, was dated as 35, 750±1, 350yrs. B.P. (GaK-3669) by ¹⁴C dating.
On the other hand, two pumice-fall deposits are distributed in a narrow area of the northeastern part of this plain. These deposits are named as Shikaribetsu pumice-fall 2 and 1 (Sipfa-2 and Sipfa-1), and considered to have been derived from the Shikaribetsu volcano. ¹⁴C dating investigation revealed the date of the Sipfa-2 ejection to be older than the Spfa-1 deposits dated as 32, 200±2, 000yrs. B.P.(GaK-714). Then, archaeologic investigation revealed the date of the Sipfa-1 ejection younger than the Late Paleolithic Shimaki remains dated as 19, 000yrs. B.P. using the hydration layer of obsidian artifacts.
In the latest glacial period (W IV), andesitic pumice-fall (E-a) erupted from the Eniwa volcano near the Shikotsu caldera, is chiefly distributed in the central area of this plain. Tephrochronologic investigation in this plain and Ishikari low land revealed the date of eruption of the E-a deposit to be older than about 13, 000yrs. B.P. and younger than about 15, 000yrs. B.P. On the basis of the correlation of terrace stratigraphy, periglacial phenomena and tephrochronology of these pumice-fall deposits in this plain, Table 1 is established.