九州内帯第三系の砂岩組成  [in Japanese] Petrography of the Tertiary sandstones from the Inner Zone of Kyushu, Japan  [in Japanese]

九州内帯第三系砂岩の主成分鉱物組成と重鉱物組成を検討し, 後者についてはA〜Gの7タイプを識別した。基盤岩類との不整合の直上の第三系最下部層は, 基盤岩中と同種類の重鉱物を高頻度で含み, 石英に乏しく岩片に富む。厚い地層を持つ規模の大きな堆積盆地においては, 第三系の中・上部ではジルコン, 電気石, ルチルなどの安定重鉱物と石英を多く含み, 重鉱物組成の層位的な変化は著しくはない。これに対して小規模堆積盆地では, 第三系の中・上部でも重鉱物組成は層位的に顕著に変化する。これらの鉱物組成の変化傾向は泥質岩の化学組成の変化傾向とも調和的で, 堆積盆地の規模や発達経過が堆積物の性質に反映された結果とみなすことができる。

Mineral assemblages of the Tertiary sandstones from the Inner Zone of Kyushu were examined. Heavy mineral assemblages of the sandstones are classified into seven types. Vertical and lateral varieties in lithology and thickness of the sedimentary sequences are well reflected in the changes of the heavy mineral assemblages. Heavy mineral assemblages in the lowest parts of the Tertiary sequences are similar with those in the underlying basement rocks. The stratigraphic distribution of the assemblages in the main parts of the sequencesa remarkedly varying in each basin where thin sedimentary sequence has accumulated. In contrast, changes in the main part of thick formations in large-scale basins are not pronounced: uniformity of the assemblages indicates that the basins were rather large and/or the geology of the surrounding land areas were monotonous, and consequently the thick sediments accumulated with little change in the heavy mineral assemblages. Stratigraphic changes in the major constituents of sandstones are in harmony with those of the heavy mineral assemblages. Light and heavy mineral analyses of sediments are useful methods for the provenance elucidation and determination of styles of basin development.