四国西部の白亜系四万十累層群の砂岩化学組成と四万十帯北帯における岩石相ユニットの対比  [in Japanese] Chemical composition of sandstones from the Cretaceous Shimanto Belt, western Shikoku, Japan, and correlation of petrofacies units in the Northern Shimanto Belt  [in Japanese]

四国西部の白亜系四万十帯は, 砂岩の化学組成上の特徴から2つのユニットに区分される.この2つのユニットは, 組成的に四国東部の年代的に古いKS Iユニットと新しいKS IIユニットにそれぞれ類似するが, 相違点も認められる.そこで, 四国西部の白亜系四万十帯をKS I'とKS II'に区分した.九州東部の白亜系四万十帯砂岩の化学組成は, 四国西部の砂岩のそれに似ている.また, 赤石山地の白亜系四万十帯砂岩は, 四国東部の砂岩に似ている.すなわち, 九州東部の砂岩はKS I'とKS II'に, 赤石山地の砂岩はKS IとKS IIに区分可能である.KS I'-KS II'とKS I-KS IIとの境界は, ほぼ四国中央部に存在すると推定される.また, KS I'とKS II', およびKS IとKS IIとの境界年代は, 九州から赤石山地に向かい若くなる.

The Cretaceous Shimanto Belt in western Shikoku can be divided into two petrofacies units on the basis of sandstone geochemistry. The two petrofacies units are geochemically similar to and are chronologically correlative with KS I (younger) unit and KS II (older) unit in eastern Shikoku, respectively, although there are some differences in chemical signature. So we designate the two units in western Shikoku as KS I' and KS II'.Sandstones from the Cretaceous Shimanto Supergroup in eastern Kyushu are geochemically similar to those in western Shikoku, and those in the Akaishi Mountains to those of eastern Shikoku, respectively. This means that the Cretaceous Shimanto Belt in eastern Kyushu can also be divided into KS I' and KS II' units, and the belt in Akaishi Mountains into KS I and KS II units. The boundary between KS I'-II' units and KS I-II units is supposed to be around central Shikoku.The ages of the boundaries between KS I' unit and KS II' unit, and between KS I unit and KS II unit become younger eastward from Kyushu to the Akaishi Mountains.