びわ湖周辺の第四系とびわ湖ボーリングサンプルの観察結果についておよび火山灰分析結果

Abstract

It is significant in Quaternary geology of Japan that a 200m core sample was obtained from the bottom of Lake Biwa-ko. The core is mainly composed of clayed sediments including a number of inserted volcanic ash layers. The geohistory of late-Quaternary period in Lake Biwa-ko may be reconstructed firstly by the success of this boring and complete investigations of the core samples, which may cover surely the whole time since 0.5 million years ago. Up to the present time, no geologic research of the surrounding hill lands has been successful to establish a whole geohistory in the area in question. The Quaternary system around Lake Biwa-ko is roughly divided into three units : Plio-Pleistocene Kobiwako Group, Pleistocene Terrace Deposits and Alluvium. These sediments do not continue through whole ages, because they are accumulated uncomfortably each other. The 200m core sample, however, consists of a continuous series of sediments. It is extremely useful for the reconstruction of whole geohistory of ancient Lake Biwa-ko in the late Quaternary. In this paper, the writer deals with the characteristics of volcanic ashes found in the upper part of the 200m core sample and discusses the correlation between those ashes and the volcanic ash layers in the Kobiwako Group. The volcanic materials in the core are divided into two types, andesitic and trachytic. The former contains two pyroxenes, hornblende and abundant volcanic glass flakes as the constituent minerals. The latter is chiefly composed of orthorhombic pyroxene, hornblende, biotite, quartz, volcanic glass flakes and small pumice grains. The individual volcanic seam is distinguishable from the differences in some features as shown in Tables 2 and 3. The writer hopes to establish the tephrochronology of whole Quaternary deposits around Lake Biwa-ko by making clear the characteristics of volcanic materials in this core sample and in the Kobiwako Group. It is the writer's expectation that geohistory of Lake Biwa-ko will become wealthy by the investigations carried out on the basis of tephrochronology.