南部九州長島火山岩類の層序と放射年代

概要

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The Nagashima volcanic rocks are distributed in the Nagashima island and its adjacent areas west of southern Kyushu, and are subdivided into the Sashikise, Kuronoseto, Hoya and Miyanoura Formations in ascending order. The Sashikise Formation unconformably overlays the basement rocks consisting of sandstone, mudstone and conglomerate of the Cretaceous and Paleogene. The formation is mainly composed of the Narusebana welded tuff. The Kuronoseto Formation exposed in the southern area of Nagashima island consists of tholeiitic andesite. The Hoya Formation is divided into the lower and upper members. The lower member consists of conglomerate containing many cobbles of tholeiitic andesite, lapilli tuff and pumice flow deposits and the upper member named Gyonin dake andesite is composed of hornblende andesitic lava and tuff breccia. The Miyanoura Formation is subdivided into two members. The lower member consists of tuff breccia, lapilli tuff, and tuff, and the upper member called Ohama andesite is composed of pyroxene andesitic lava accompanied with tuff breccia. These formations show unconformable relationships with each other. The geologic ages of the Narusebana welded tuff and Gyonindake andesite are determined 3.2±0.3 Ma by fission-track method and 2.5±0.3 Ma by K-Ar method, respectively. From these dates it can be concluded that the volcanism in the Nagashima island took place during Pliocene time.
日本地質学会の論文
1984-11-30