北海道石狩低地帯周辺の更新統

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The Ishikari Lowland is a significant boundary which divides geotectonically the Hokkaido island into the two parts: the Green Tuff area where Tertiary volcanics are distributed, and the Hidaka-Kamuikotan Belts which consist mainly of pre-Tertiary sediments and metamorphic rocks. This lowland has been an active subsidence basin since Late Cenozoic time. The Quaternary deposits in the Ishikari Lowland are distributed in and around the Ishikari Hills, Nopporo Hills, Umaoi Hills and their subsurface. In this paper, we review the stratigraphy, biostratigraphy, and chronological setting of Pleistocene deposits. The stratigraphie frame is shown in Fig. 2. The following acts have been acertained in a process of examination on Quaternary stratigraphy in the Ishikari Lowland. 1) No information on the Pliocene-Pleistocene boundary is available in this lowland. 2) Fission-track dating in the upper of the Uranosawa Formation has revealed to be 1.46±0.23 Ma. Althrough the base of the Uranosawa Formation can not be observed up to now, this result explains that it corresponds to the Early Pleistocene epoch. 3) According to the biostratigraphic data, the last stage of the Early Pleistocene (around the Jaramillo Event?) involves the most distinct episode of the floral and faunal change. 4) Four stages of marine transgression occurred on the Ishikari Lowland in the Pleistocene time. The transgression was from the Early Pleistocene to the early Middle Pleistocene (Uranosawa-Shimonopporo stage), in the Middle Pleistocene (Otoebetsugawa Stage), in the Last Interglaceal Age (Momijidai-Atsuma Stage), and in subinterstadials. 5) The paleoclimatic reconstruction of the Pleistocene time based on the palynological studies suggests the existences of the three cold phases (the late Early Pleistocene, the Shizukawa Cold Age "Sz-III horizon", and the Last Glacial Age) and two warm phase (Otoebetsugawa-Hayakita Stage, Momijidai-Atsuma stage). The paleoclimate of the Momijidai-Atsuma stage which corresponds to the Last Interglacial Age, was almost the same as the present climate, and within it one short cool episode was also detected. Two short epidsodes of relatively tempelate climate also occurred in the Last Glacial Age.
日本地質学会の論文
1988-04-25

北海道石狩低地帯周辺の更新統

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