埼玉県立自然史博物館研究報告 第５号 (1987)
岩手山麓に分布する渋民溶結凝灰岩の火山灰層序と
フィッション・トラック年代
町 田 瑞 男
埼玉県立自然史博物館
大 上 和 良
岩手大学工学部資源開発工学科
土 井 宣 夫
日本重化学工業（株）
畑 村 政 行
滝沢村役場
ま と め
岩手山麓，北上山地北西縁部および北部北上低地帯に分布する渋民溶結凝灰岩について検討した結果，次のことが明らかになった．
1．渋民溶結凝灰岩は噴出時の地形に沿って分布し，岩手山麓から北上低地帯では地表下45〜230mに，東方にむかって分布高度が高くなり，北上山地内では標高550m前後まで確認される．
2．分布の中央部では強溶結し，残丘斜面を薄く（2〜3m）覆う場合においても溶結している．北にむかって溶結度が低下し，非溶結相となり，これらは漸移的に移行する．
3．岩手山北麓の試錐試料をもとに命名された西根溶結凝灰岩は，渋民溶結凝灰岩の強溶結部で，両者は同一の火砕流堆積物である．
4．今回，渋民溶結凝灰岩の黒色ガラスレンズを試料としたフィッショントラック法で得られた年代は，（71．6土6.2）×104yrsで，前期更新世を示す．この年代は，K−Ar 法で得られた年代と矛盾しない．また，本岩の示す逆帯磁期は松山逆磁極期に対比される．
5．渋民溶結凝灰岩の下位に堆積している岩手川口・玉山および寺林火山灰の各累層は前期更新世のものであることが判明した．同時に，七時雨火山起源の沼宮内火山泥流堆積物も前期更新世の堆積物である．
6．渋民溶結凝灰岩は広域に分布することから，今後前期更新世における地層の対比および地史の復元に重要な鍵層となる．
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第3図 渋民溶結凝灰岩の高度分布図 （部分）
第2図中の断面a−bに沿い，その北および南側のそれぞれ3kmの範囲内に分布する渋民溶結凝灰岩の分布高度を投影した．個々の高度分布は枠で示した範囲内におけるものである．
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Tephrochronology and fission track age of the Shibutami Welded
Tuff distributed around Iwate Volcano, Northeast Honshu, Japan
Mizuo MACHIDA
Saitama Museum of Natural History, Nagatoro, Chichibu-gun, Saitama Pref., 369-13 Japan
Kazuyoshi OKAMI
Department of Mining and Civil Engineering, Faculty of Technology, Iwate University, Iwate Pref., 020 Japan
Nobuo DOI
Japan Metals and Chemicals Co. Ltd., Iwate Pref., 020-01 Japan
and
Masayuki HATAMURA
Takizawa Village Office, Iwate-gun, Iwate Pref., 020-01 Japan
Abstract The volcanic ashes, which attain 70 meters thick, distributed around the eastern foot of lwate Volcano are composed of nine beds. The Shibutami Welded Tuff, the main studied rock body, was intercalated between the Iwatekawaguchi and Ekarinai Volcanic Ash Beds which are the third and the forth beds from the lower (OKAMl et al., 1980).
The Shibutami Welded Tuft is dacitic tuff which includes the phenocrysts of plagioclase mainly, hypersthene, augite, quartz and includes rarely hornblende, and judging from the existence of pumice fall and volcanic ash beds just under the tuff, the volcanic eruption shows the change of pyroclastic fan into pyroclastic flow. The distribution of the Shibutami Welded Tuff is parallel with configuration of the ground at the time of the eruption. And the degree of welding of the tuff is the highest near Shibutami Village at the foot of lwate Volcano and gradually becomes weak northward and finally becomes non-welded at Numakunai. Iwate Town in the northernmost part of the Kitakami Lowland Belt.
The lithofacies and chemical composition of the densely welded Shibutami Welded Tuff intimately resembled the Nishine Welded Tuff (TACHIBANA, 1971) named for the boring column material, drilled at the northern foot of Iwate Volcano. The Nishine Welded Tuff has a weakly welded part in a upper part, which is popular facies in the Shibutami Welded Tuff. And so the both welded tuffs are the same pyroclastic flow deposits.
The black-colored volcanic glass lenses (5mm thick and one to several cm length) are intercalated in the lower part of the dense welded part of the Shibutami Welded Tuff and one of them is used for fission track dating material. The obtained fission track age is (71.6 ±6.2) x 104yrs, which is more or less younger than the age obtained by K-Ar dating method. The Shibutami Waded Tuff is in the state of dimagnetization, and the tuff may be erupted in the Matsuyama Reverse Epoch seeing from the obtained age.