要　旨　埼玉県秩父郡大滝村の王冠地域には，後期中新世の火山性陥没盆地が存在する．規模は小さいが，陥没した後に溶結凝灰岩をともなう陸上火山活動が生じている．陥没地内を埋積する王冠層は，周辺部の高角不整合面に対してアバットの関係で堆積している．
　大冠層が堆積した後期中新世の関東地方は，全般的隆起の時期にあたっており，隆起運動とともに，群馬県の本宿をはじめとして各地で激しい陸上火山活動が発生している．王冠地域のものもこうした活動の具体的なあらわれである．
　王冠地域の南南西方向にあたる三富・太良ケ峠地域にも火山性縮没盆地が知られており，深成岩体や陥没盆地の長軸の方向などから一つの群れをなす陥没盆地であるとみられる．王冠地域の火山性陥没盆地は，地下でのマグマ群の動きを推定する上で，重要な位置を占めるものと考えられる．

キーワード：後期中新世，火山性陥没盆地，王冠層，安山岩質火砕岩，溶結凝灰岩，関東山地

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A Late Miocene Volcanic Collapse Basin in the Okaburi District, Saitama Prefecture, Central Japan

Hisao Adachi
Shinobugaoka Highschool. 5-1-20 Asakusabashi, Taito-ku, Tokyo 111-0053
Kiyoshi Koizumi
Eifuku Highschool. 1-7-28 Eifuku, Suginami-ku, Tokyo 168-0064
Yuichi Okano
Hanno Highschool. 17-13 Honmachi, Hanno-shi, Saitama 357-0034
and
Yoshiaki lshida
Hitotsubashi Highschool. 1-12-13 Higashikanda, Chiyoda-ku, Tokyo 101-0031

Abstract A late Miocene volcanic collapse basin located in the Okaburi district in Otaki-mura, Chichibu-gun, Saitama Prefecture. The collapse was followed by small-scale terrestrial volcanism accompanied by welded tuff. The Okaburi Formation aggraded into the collapsed basin and deposited on a high angle unconfomity that forms an abutting contact with the basement.
The late Miocene was an age of general uplifting in the Kanto region, accompanied by violent terrestrial volcanism at a number of districts, including Motojuku in Gunma Prefecture. The Okaburi district also shows evidences of this activity.
Volcanic collapse basins are also known in the Mitomi and Taragatoge areas on the south-southwest side of the Okaburi district. The major axes of these collapse basins and those of the plutonic rock bodies are oriented NNE-SSW so they are all believed to be elements of the same group. The Okaburi Fomation is important in determining of the movement of magma swarm.