北海道苫小牧市静川ボーリング・コアにみられる第四系

抄録

A stratigraphic study was made on the Quaternary deposits in the borehole at Shizukawa, situated in the southeastern marginal part of the Sapporo-Tomakomai Lowland in Hokkaido. The core of the borehole is divided lithostratigraphically into 8 formations, namely SZ-I to SZ-VIII Formation in ascending order.
Each formation is characterized, apart from its lithology, by molluscan fossils, pollen assemblage, palaeomagnetic property and tephra identification (Fig. 3, Table 3).
The results of the study are summarized as follows:
1) SZ-I Formation is possibly correlated with the Lower Pleistocene, because the horizons of reversed magnetic polarities are dominant in the Formation. Pollen assemblage indicates that the sedimentation occurred under a cooler climatic condition than that of the present age in this area.
2) SZ-II and SZ-III Formations, deposited during the Brunhes Normal Epoch, are probably correlated with the Middle Pleistocene. SZ-II Formation, mainly consisting of marine sediments, deposited during the time range when it was somewhat warmer and more humid than today. The Formation is reasonably correlated with the Hayakita Formation distributed in the Atsuma area, on the basis of common pollen assemblages in both formations. The geologic age of the Formation is provisionally correlated with the M/R Interglacial Age.
SZ-III Formation intercalates peaty beds which indicate the existence of a boreal forest. The authors will call the age of the deposition of the beds “Shizukawa Cold Age”. The age may be correlated with the Riss Glacial Age.
3) SZ-IV Formation, having some marine deposits in the lower part, may have been deposited under a similar climatic condition to that of the present age. The Formation is possibly correlated with the Atsuma Formation on the basis of the features of its pollen assemblages. The range of the Formation may be correlated with the early Late Pleistocene, namely the R/W (Last) Interglacial Age.
4) The time range of the three formations, namely SZ-V, SZ-VI and SZ-VII, is probably correlated to the Würm (Last) Glacial Age. SZ-VI Formation, including the same key tephras, namely the Azuma ash fall deposits (>43, 200+4, 300-2, 700y.B.P.), is reasonably correlated to the Hongo Formation which has been considered as the deposits during the early Würmian Glacial Age. SZ-VII Formation intercalates the Shikotsu pumice fall deposits (from about 35, 750±1, 350y.B.P. to 32, 200+4, 700-3, 100y.B.P.). The time range of the SZ-V Formation is presumed to be almost the same as that of SZ-VI Formation on the basis of its stratigraphic position.
5) SZ-VIII Formation is the Holocene.