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.
In the present article the authors intended to make clear the geologic age of the Chuetsu and Uonuma Groups, through the studies on the stratigraphy and the pollen floral change of these two groups. The results obtained are summerized below. 1) The geology of the Chuetsu and Uonuma Groups in the western area of Tokamachi city is shown in Fig. 1. 2) The results of the pollen analysis are shown in Figs. 2 and 3. The prosperity and decline of Taxodiaceae are well shown in these figures, and the floral zonation in Fig. 4 is established based on the occurrence of Taxodiaceae. 3) The curve of the paleo-temperature in Fig. 4 was drawn upon the ecological data of the pollen assemblages. 4) In Fig. 5, the Uonuma Group is compared with the Osaka Group which shows the most representative Quarternary succession in Japan, mainly based on the pollen floral change. 5) The occurrence of the genera of Taxodiaceae is very characteristic: namely they change from (1) inferiority of Metasequoia, to (2) flourish of Metasequoia, to (3) inferiority of Metasequoia with Cryptomeria, to (4) flourish of Cryptomeria ascendingly. This phenomenon is samely seen in the Osaka Goup. 6) The disappearance of the Tertiary elements and the appearance of the Quarternary elements in the Uonuma Group are earlier than in the Osaka Group. 7) If we permit the presumption that the horizon of so-called Olduvai normal events is the boundary of Plio-Pleistocene, the boundary concerned in the Uonuma Group may be in the neighbourhood of Mf-3.
Since Emiliani et al. (1961) analysed the paleotemperature of the Pliocene and Pleistocene in le Castella area, southern Italy, the proposed type locality of the boundary between the Pliocene and Pleistocene, more-detailed works had begun on the stratigraphic problems of the Upper Pliocene and Lower Pleistocene in various-areas of the world. Among the recent knowledge, the combined microbiostratigraphy and magnetic chronology is the most useful criteria for the Pliocene and Pleistocene correlation. The authors undertook a tentative correlation of the Upper Pliocene and Lower Pleistocene between le Castella area and the Boso Peninsula (Fig. 2) based on their previous studies of the magnetic stratigraphy made on both areas (Nakagawa, Niitsuma, and Hayasaka, 1969; Nakagawa, Niitsuma, and Elmi, 1970). The base of the Calabrian in le Castella area could be placed at a horizon younger than the Olduvai Event in the Matuyama Reversed Polarity Epoch in the magnetic stratigraphy in the sense of Hays et al. (1969). Designation of the type locality of the Pliocene-Pleistocene boundary and the microbiostratigraphic evidence of the horizon around the boundary were previously reviewed briefly.