Abstract
The ancient sand dune deposits in the Tokachi plain, east Hokkaido, which were constructed by reworked pyroclastic fall materials as a result of the eolian flowing, was studied. These deposits, 1-10m in thickness, are mainly composed of pumiceous fine sand, being covered by the younger volcanic ashes and soils. More than 300 dunes are distributed on the Pleistocene terraces named as “Kamisatsunai I Plain” (Fig. 4).
Tephrochronologic and topographic investigation revealed the date of the dune sands to be older than the Tarumai pumice-fall deposit “d” (T-d) in about 9, 000 years B.P. and younger than the Shikotsu pumice-fall deposit 1 (Spfa-1) dated as 32, 000 years B.P. (Fig. 1). The ancient dunes can be classified into three types from the view point of stratigraphic features in the deposits; namely, the first type dune derived from the Spfa-1 deposit, the second type dune derived from the Spfa-1 and Eniwa pumice-fall deposit “a” (E-a), and the third type dune, the newest one of them, derived from the E-a deposit. The third type of them is predominated in numbers.
As shown in Figs. 4, 5 and 6, most of them had an oval form, and the long-axis pointed to the northwest. This may give an important information on the prevailing wind in the past. They are supplied with sand carried by the prevailing wind from the unconsolidated pyroclastic deposits (E-a and Spfa-1) covered with poor vegetation, probably under the arid condition. Accordingly, the formation of those ancient dune in this plain might have indicated the palaeoclimatic change in the end of Würm glacial age.
