宮崎層群に挟在する凝灰岩層とそのK-Ar年代

概要

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The Mio-Pliocene Miyazaki Group in the Miyazaki coastal plain, southern Kyushu consists mainly of marine sediments which have well preserved tuff beds. Stratigraphy, lithology and petrography of the tuff beds were investigated. The tuff beds have mainly heavy mineral compositions of hornblende-orthopyroxene-opacity minerals, hornblend-orthopyroxene-clinopyroxene-opacity minerals and orthopyroxene-clinopyroxen-opacity minerals. The glass shards contained in these tuff beds consist of main sponge and fiber types, and minor Y and plate types. The representative 8 tuff beds as key bed in the studied area were dated by K-Ar method using mineral separates of plagioclase, biotite and hornblende, giving 6.5 to 2.0 Ma, being consistent with the biostratigraphy based on planktic foraminifera. The frequency of the tuff beds intercalated in the Miyazaki Group increases with time, having a maximum around 3.0 Ma. A clinopyroxene component of heavy mineral composition tends to increase after 3.5 Ma. The first appearance of seoria bed was confirmed in the upper part of the Group, being ca. 2.4 Ma in age. These characteristics for the tuff beds resemble with those of volcanic rocks distributed in the Hokusatsu area.
特定非営利活動法人日本火山学会の論文
2000-07-10