鉱物組成・化学組成からみた和泉砂岩  [in Japanese] Mineral and chemical compositions of the Izumi sandstones, Southwest Japan  [in Japanese]

松山地域, 砥部地域, 池田地域, 引田地域, および淡路地域の5地域における上部白亜系和泉層群のうち, 炭質物や石灰質セメントを含まない塊状中粒砂岩の鉱物組成と化学組成を調べた。これらの砂岩は主として, 石英, 斜長石, カリ長石, および岩石片から成り, 岩石片は, 変質したガラス質流紋岩, 流紋岩質凝灰岩, チャートなどである。より下位の層準が露出する松山地域の砂岩は石英粒子を多く含み, 池田地域のものには斜長石が多いというわずかな違いがあるものの, 大局的には, 地域差はほとんど認められない。和泉砂岩の化学組成をみると, シリカ含有量は松山地域のより粗粒な砂岩で高く, 池田地域のより細粒の砂岩で低い。これは岩石片と基質の量によるものと思われる。しかし, アルカリやアルミナなどの量は, 粘土基質の量と鉱物組成との関わりが大きいと思われる。

Mineral and chemical compositions of the Upper Cretaceous Izumi sandstones were investigated in five districts, Matsuyama, Tobe, Ikeda, Hiketa, and Awaji districts. Analysed samples are mostly massive, medium-grained sandstones free from carbonaceous matter and carbonate cement. The Izumi sandstones are composed mainly of quartz, plagioclase and potash feldspars, and rock fragments. Rock fragments comprise altered glassy rhyolite, rhyolitic tuffs, and chert. There exists a minor variation in that the sandstones of the Matsuyama district (in the lower horizons) contain more quartz grains while those of the Ikeda district contain more plagioclase grains. Relationships between chemical and mineral compositions were detected. Silica contents of the coarser-grained sandstones of the Matsuyama district are higher than those of the finer-grained sandstones of the Ikeda district, which may be due to the amount of rock fragments and matrix. Other components such as alkalies and alumina, however, do not relate to the mineral composition of framework grains. This fact implies that they relate to the amount and mineralogy of the clay matrix. Little areal variation in mineral and chemical compositions are recognized. This fact suggests that the sediments of the Izumi sandstones were deposited without a significant change in the provenance and transportation mechanism.