十勝層群の層序と年代

概要

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The Tokachi Group, developing from Pliocene to Early Pleistocene, buried the Tokachi Tectonic Basin formed during the Pliocene in the Tokachi area, southeast of central Hokkaido. Recently, this Group has become known as a result of researches on litho-stratigraphy, volcano-stratigraphy, bio-stratigraphy, magnetic-stratigraphy, radioisotope stratigraphy and tectonic development. Based on these studies, this paper summarizes the stratigraphy and chronology of the Tokachi Group and discusses related problems. 1. In each horizon of the Tokachi Group, there exist characteristic beds of volcanic ash, pumice fall and pumice flow. In the present study, these beds were studied from various aspeets such as stratigraphical position, lithological and paleo-magnetic characteristics and radioisotope stratigraphy; and then classified into 7 horizons cf tephra as shown in fig. 1. 2. By studying these tephra as key beds, the Tokachi Group can be divided into four formations - Honbetsu (Nukanai), Ikeda, Osarushinai and Shibusan Formations - from lower to upper level. Relationships between formations were discussed as follows: 1) Honbetsu (Nukanai) and Taiki Formations exhibited clino-unconformity at Makubetsu Plateau. However, since the Taiki Formation belongs to the Thalassiosira oestrupii Zone (3.70-5.10 Ma) of diatom-bio-stratigraphy, whereas the basal pyroclastics of the Honbetsu (Nukanai) Formation contains Pliocene molluscan fauna, it is considered that the relationship between the Taiki Formation and the middle and lower Honbetsu Formation is of contemporaneous heterotopic facies. This fact indicates that the later formed pertions of the Tokachi Tectonic Basin overlaid the pelagic sedimentary basin which had continuously accumulated the Taiki Formation in the early Pliocene. 2) A formation mainly consisting of pyroclastics developed in north-east Tokachi district-traditionally called the Ashoro Formation - is related via contemporaneous heterotopic facies with the upper Honbetsu Formation and also with the middle/lower Ikeda Formation. 3) At Makubetsu Plateau and Osarushinai Hill, the Osarushinai Formation covers the erosion surface of the Ikeda. Formation; at north-east Oribe Plateau, the lower Osarushinai Formation does not exist and its upper formation - the Kiyosumi volcanic ash horizon-develops and directly comes into contact with the Ikeda Formation. 4) Distribution of the Shibusan Formation, the uppermost member of the Tokachi Group, is essentially limited to the present Obihiro basin. The basal Shibusan Formation, Kuttari-Meto tuff bed, covers the Osarushinai Formation with a prominent unconformity at the west margin of the Oribe-Osarushinai-Makubetsu plateau. 3. The Plio-Pleistocene boundary in the Tokachi district is assumed to exist at the boundary between the Ikeda and Osarushinai Formations. The Honbetsu-Ikeda Formations are characterized by Takikawa/Honbetsu fauna represented by Fortipecten takahayashii, and the Osarushinai Formationis characterized by "Setana fauna". Thus, the boundary between both fauna corresponds to that between the Ikeda/Osarushinai Formations. 4. Development and transition of the Tokachi Tectonic Basin from the Pliocene to Early-Pleistocene is as follows: each formation of the Tokachi Group transit, in turn, its maximum thickness (maximum sedimentation area) from east to west, from lower to upper, with each formation accumullating a second order tectonic basin-Honbetsu, Ikeda, Osarushinai and Shibusan tectonic basins. Therefore, at the central area of the Tokachi Tectonic Basin, the upper and lower formations lay in conformably. However, at the marginal areas, lack of members and local-unconformity are caused by changes such as transitions of tectonic basins, conversion from sedimental areas to uplifting areas, and growth of anticlinal structures. Subsidence of the Tokachi Tectonic Basin ended in the Middle-Pleistocene, at approximately 50 Ma.
日本地質学会の論文
1988-04-25

著者

松井愈
北星学園大学

十勝層群の層序と年代

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