宮城県鳴子盆地の地形発達史

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タイトル別名
  • GEOMORPHOLOGICAL DEVELOPMENT OF THE NARUGO BASIN, MIYAGI PREFECTURE, NORTHEASTERN JAPAN
  • ミヤギケン ナルコ ボンチ ノ チケイ ハッタツシ

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The existence of paleo-lake basins in the so-called “Green-tuff Region” of northeastern Japan, has been pointed out by N. Kitamura (1959), and by K. Mitanl (1964). It is said that most of these paleo-lake basins were formed by the intensive crustal movements during Miocene and Pliocene times.<br> The present author assumes that there has been a close relationship between the existence of such paleo-lake basins and the Pleistocene volcanisms ; the present landforms have been influenced directly by the crustal movements and the volcanisms. And so, it is very necessary and important for the author to clarify the geomorphological development of these paleo-lake basin area in the volcanic region.<br> In this paper, the author took the Narugo Basin, one of the paleo-lake basins mentioned above, in order to clarify the influences of the geotectonic movements and of the volcanisms, and he tried to analyse the formation of the topographical characteristics in the basin and it's vicinity.<br> The results are summarized as follows:<br>1) The Narugo Basin was formed by the combination and the repetition of crustal movements in late Tertiary and early Quaternary, and the violent volcanisms such as the explosion of the vast pyroclastic deposits (Pleistocene Uwahara Dacites), spread over the eastern part of the basin fringe. These volcanisms seem to have been guided by the pre-existing active tectonic lines (Some of these are named “Honjo-Matsushima Geotectonic Line” by Oide and Onuma, 1960). Therefore, the present author assumes the formation of the present Narugo Basin to be a “volcano-tectonic depression (Williams, H. 1941)”.<br>2) The Narugo lacustrine sediments were deposited in the Narugo Paleo-Lake which was formed by the above mentioned movements. Theses sediments can be classified into two ; loose conglomerate and sand beds of the marginal f acies, and well laminated tuff aceous siltstone and sandstone beds of the central f acies. They imply former violent dacitic volcanism.<br>3) After several upheavals of the whole region, several levels of erosional surfaces and fluvial terrace surfaces were formed ; in late Pleistocene, the Narugo Paleo-Lake became shallow due to the progressive accumulation of fluvial deposits and the upheaval of the whole region. Soon after, the Eai Paleo-River began to incise itself into the bottom of the basin, which formed the Sanjo Surface (the first terrace), and the Narugo Paleo-Lake disappeared.<br>4) The Narugo Volcano, situated in the west center of the basin, started it's activity under fluviatile-lake environments and formed tholoide to aspire type volcanoes. This volcanic activity continued until the formation of the Daigakunojo Surface (the second terrace).<br>5) It is inferred from the character of the deposits, their distribution, and the inclination of the Sanjo Surface, that there were comparatively new geotectonic movements during and after the formation of the terrace surface. The author assumes the existence of the “Nuruyu-Nagashida Geotectonic Line” for the central axisis of these movements. The western part divided by this geotectonic line, shows on elevating tendency, while the eastern part shows a subsiding tendency.<br>6) Judging from the character of the deposits (including peaty materials) and the longitudinal profile of the Daigakunojo Surface, which submerges under the alluvial surface outside the basin, it seemes to have been formed during or soon after the Würm Ice Age.

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  • 地理学評論

    地理学評論 39 (8), 521-537, 1966

    公益社団法人 日本地理学会

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