The Uma-oi Hills and the Yuni-Abira Basin are arranged with a N-S to NNW-SSE trend on the eastern marginal area of the Ishikari Lowland, Central Hokkaido. In this paper, we discussed the stratigraphic and tectonic setting of the deposits distributed in the Yuni-Abira Basin, which were originally denned as the Upper Pleistocene Abira Formation. The Abira Formation is lithostratigraphically divided into four members, the Member I to IV in ascending order. The Member I is mainly composed of marine sediments, correlated with the Atsuma Formation in the southern coastal plain. The Member II consists of thick fan-gravels which were derived from the Yubari Mountains. The Member III and IV show mostly lacustrine facies, associated with intercalation of peaty layers. The Member IV intercalates the Shikotsu Pumice Fall Deposits (Spfa-10 to 3). Based on the palynological studies, the climatic change during deposition of the Abira Formation (Late Pleistocene time) was reconstructed. After the Last Interglacial transgression (the Atsuma Stage), a lake basin appeared in the tectonic depression of the Yuni-Abira Basin. The lake basin was gradually filled by sediments and disappeared in the early Last Glacial Age. The crustal movement since Neogene was still active in the latest Quaternary time and it contributed to the geographic change of the Yuni-Abira Basin and adjacent area.
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