Abstract
Lacustrine deposits and terrestrial volcanic products of Pliocene to Middle Pleistocene age are widely distributed in the eastern part of Toya Lake. Based on some unconformities, Pliocene and Pleistocene deposits are divided into the following major divisions: the Kitayuzawa Formation, the Rerukomabetsu Formation, the Andesite Member, the Sohbetsu pumice flow deposit and the Takinoue welded tuff, the Kimobetsu welded tuff, and the Tokushunbetsu clay member. Lower to Middle Pliocene deposits of the Kitayuzawa Formation were accumulated in two polygonal collapse sub-basins, which form the so-called Kitayuzawa basin. This basin was changed to a tectonic basin in Middle Pliocene. Upper Pliocene and Pleistocene deposits, which are represented by the Rerukomabetsu Formation, the Andesite Member and the Tokushunbetsu clay member, were piled into the valley and the dammed lake of the Osarugawa tectonic basin. Three magnetic polarity zones, namely: the lower-predominantly reversed, the middle-normal and the upper-reversed, could be recognized on the basis of paleomagnetic analysis of the Kitayuzawa and Rerukomabetsu Formations. From bottom to the top, these polarity zones are believed to correspond to the late Gilbert reversed, the Gauss normal and the early Matuyama reversed polarity chrons, respectively. Fossil pollen assemblage of the upper part of the Kitayuzawa Formation is characterized by high concentration of Picea which indicates a cooler climate than at the present. The Rerukomabetsu Formation is divided into 9 pollen zones: RK-I〜IX in ascending order. RK-I and RK-II Zones are dominated by Taxodiaceae indicating a warmer and moister climatic condition. In RK-III Zone, Picea increases but Taxodiaceae decreases. RK-IV Zone is characterized by high concentration of Picea, the presence of Selaginella selaginoides and a variety of herbs, and thus indicates a cold dry climate. Zones RK-V, RK-VII and RK-IX show high concentration of Picea, whereas Zones RK-VI and RK-VIII show high concentration of Taxodiaceae. It indicates that climatic changes occurred repeatedly. The two pollen zones, namely: TS-I and TS-II, inside the Tokushunbetsu clay member represent climatic change from mild to cooler than the present. Based on the radiometric ages, magnetostratigraphy and pollen assemblages, RK-I and II Zones of the Rerukomabetsu Formation may be correlated to the upper part of the Horokura Sandstone-Mudstone Member of the Lower Pliocene Fukagawa Group in Central Hokkaido. The climatic change from warm-wet in Zone RK-II to the cool and cooler-dry in Zones RK-III and RK-IV may be correlated with that recognized at the boundary (which also represents the Gauss/Matuyama boundary) between the lower and upper parts of the lower part of the Ikeda Formation which belongs to the Tokachi Group in Eastern Hokkaido. The climate recorded in the Tokushunbetsu clay member is correlated to cool stage of Middle Pleistocene, detected in the upper part of the Otoebetsugawa Formation distributed in the Ishikari Lowland.
