岩手県花巻市西部の鮮新・更新両統(予報)

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概要

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• The Pliocene and Pleistocene strata distributed in the southwestern environs of Hanamaki City, Iwate Prefecture are subdivided lithologically into six rock units by the writers. They are as follows in ascending order ; Wagagawa, Mitsusawagawa, Shittaigawa, Shibabayashi, Toyawaki Formations and the fan gravels. The former three formations belong to the Pliocene and the latter to the Pleistocene. The Wagagawa Formation comprises tuffaceous conglomerate, sandstone and siltstone with intercalations of thin layers of lignite and lignitic shale. It has two Ostrea beds and a marine Molluscabearing bed in its upper part. The Mitsusawagawa overlies the Wagagawa with slight disconformity, and is lignite-bearing and generally conglomeratic as a whole. This formation yields the fossil seed, nut, and cone such as of Menyanthes trifoliata, Juglans cinerea megacinerea, Picea Maximowiczii respectively. The next younger Shittaigawa Formation comprises mainly rounded and subrounded gravels ranging from cobble to pebble in size. These Pliocene strata of this area have a synclinal bending structure as a whole. The Pleistocene Shibabayashi Formation rests on the abovementioned Pliocene with unconformity and comprises mainly decomposed gravels with some intercalations of clay and peat. The latters yielded Picea jezoensis and P. Glehni. The Shibabayashi has a monoclinal structure. The Toyawaki Formation is the gravel beds intercalating clay, peat and/or peaty clay. The latters yielded Picea jezoensis, P. Glehni and Juglans manschurica hosenjiana. The Toyawaki is horizontal in structure and unconformable to the subjacent Shibabayashi. The gravels forming topographically the fan seem to cover the horizontal Toyawaki Formation. The movement which caused the deformation of the strata is separated into two phases. The first one precedes the deposition of the Pleistocene Shibabayashi and the second one is of post-Shibabayashi and pre-Toyawaki time. The latter has the same characteristic as the movement of the Detana flexure (MURATA, 1939) and also with that of the Isawa-Yushima flexure (NAKAGAWA et al., 1963) in the adjoining southern region.

• 地学団体研究会の論文
• 1966-11-30

著者

• 岩井 淳一

  東北大学理学部地質学古生物学教室

• 木下 尚

  岩手県立盛岡第一高等学校

関連論文

• 八戸-久慈地区(チリ地震津波による三陸沿岸被災地の地質学的調査報告)
• 北上川中流沿岸の第四系および地形:北上川流域の第四紀地史(2)
• 岩手県花巻市西部の鮮新・更新両統(予報)
• 福島県平市で発見された更新世植物化石

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