著者

松井 愈 松澤 逸巳

出版者

Japan Association for Quaternary Research

雑誌

第四紀研究 (ISSN:04182642)

巻号頁・発行日

vol.23, no.4, pp.233-244, 1985-01-31 (Released:2009-08-21)

参考文献数

17

被引用文献数

8 7

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The purpose of this paper is to report the stratigraphy, age and tectonic movement of the Pliocene-Late Pleistocene deposits on the Makubetsu Plateau in the central region of the Tokachi plain on the basis of the geological relations of these deposits and K-Ar age data of the pyroclastic materials and obsidian pebbles collected from 8 horizons in these deposits.1. The Tokachi Super Group, Pliocene-Early Pleistocene, distributed in this Plateau, is divided into the Ikeda and the Osarushinai Formations in ascending order. According to the molluscan and foraminiferal fauna, paleomagnetic stratigraphy and K-Ar age data, the age of the Ikeda Formation is correlative to the latest Pliocene, and the Osarushinai Formation to Early Pleistocene. The Osarushinai Formation, composed of clastic and pyroclastic materials, was accumulated in shallow sea and terrestrial basin formed subsequently by the westerly shifting of the center of the Ikeda depositional basin.2. The Makubetsu Plateau was transformed into an upheaving area after the deposition of Osarushinai Formation, while the Obihiro Basin remained as a relatively subsiding area. Along the western margin of the Makubetsu Plateau runs an active tectonic line called Oribe Fault.3. It is noticeable that the thick fanglomeratic deposits, the Kochien Gravel Bed, prevail throughout this Plateau. The lithofacies of this Gravel Bed suggest that the western hinterland, Hidaka Belt, was eroded away vigorousely in Middle Pleistocene. Some younger gravels and pyroclastic deposits cover the dissected Kochien Gravel Bed. Those younger gravel deposits are fan-surface deposits formed by erosion and resedimentation of the older thick gravel bed.4. The areal changes in the altitudes of both the upper surface and the basal level of the Kochien Gravel Bed in and around this area have been critically analyzed to detect the Late Pliocene to Holocene tectonic movements which displaced and deformed this gravel bed. It has been disclosed as a result of this study that the activity of the Oribe Fault has a right-lateral strike-slip component and shows a vertical component decreasing gradually since Early Pleistocene. Moreover, it is worth notice that the Oribe active fault appears to have originated along the boundary between the Hidaka and Tokoro belts.

著者

保柳 康一 三戸 望 吉岡 正俊 宮坂 省吾 渡辺 寧 松井 愈

出版者

地学団体研究会

雑誌

地球科學 (ISSN:03666611)

巻号頁・発行日

vol.39, no.6, pp.393-405, 1985-11-25

被引用文献数

6

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Thick Miocene sediments are distributed in the Ishikari-Teshio Belt along the western side of the Kitami mountain range in the north and the Hidaka mountain range in the south. In the southern part of this belt, the Miocene sequences are divided into the Furanui, Ukekoi, and Motokanbe Formations, in ascending order. The Furanui Formation (early Middle Miocene) consists mainly of greenish gray sandstones and mudstones The Ukekoi Formation (Middle Miocene) composed mainly of alternating beds of sandstone and mudstone and conglomerates, attains a maximum thickness of about 2,100 meters, which is correlated with the Kawabata Formation in the Ishikari district and the Kotanbetsu Formation in the Haboro district. The Motokanbe Formation (Late Miocene) consists mainly of hard shales and subordinate intercalated conglomerates. The Ukekoi Formation is subdivided lithologically into the U_1, U_2, and U_3 members in ascending order. The U_1 and U_3 are composed mainly of alternating beds of sandstone and mudstone with Bouma sequences, and the U_2 member consists of coarse clastic sediments with grading and imbrication structures developed well in the conglomerates. These facies are of turbidites and their related sediments. In the southern part of the studied area, these three members change laterally into the U4 member composed of alternating beds of sandstone and mudstone. The distribution pattern of these members can be assingned to the submarine fau environmental models of WALKER and MUTTI (1973) or WALKER (1978). The U_2 member is of the upper fan channel-fill deposits in the northern part of this area and of mid-fan deposits in the central part. The U_4 member in the southern part corresponds to the lower fan depoosits. The upper fan main channnel contains numerous smaller channels 10 meters or more deep and some 100 meters wide. These channels are filled with coarse clastic sediments transported from the north. On the basis of the above-mentioned discussion with the examination of the conglomerates, it is concluded that the Middle Miocene Ukekoi Formation is of submarine fan deposits transported far from the northern part of the Hidaka and Kamuikotan Belts, e. g. the Uenshiri Horst, while the Upper Miocene Motokanbe Formathionis of fan delta deposits derived from the eastern adjacent hinterland of the Hidaka Metamorphic Belt.