酸性凝灰岩と密接に産する砂岩の形成 : 紀伊半島四万十累層群の竜神累層千疋山酸性凝灰岩体を例として  [in Japanese] The formation of sandstones associated with acidic tuff : an example of the Chibikiyama acidic tuff in the Ryujin Formation of the Shimanto Super group, Kii Peninsula, Japan  [in Japanese]

紀伊半島四万十累帯日高川層群竜神累層(カンパニアン〜マストリヒシアン)には, 多量の酸性凝灰岩が挾在されている。それらの多くは, 薄層理で細粒であるが, 御坊市南東部の千疋山周辺には厚層理で粗粒の酸性凝灰岩が分布している(千疋山酸性凝灰岩)。その構成岩相である軽石質凝灰岩は, 内部構造の特徴から密度流で形成されたか, あるいはash flow turbiditeであると考えられる。またこの凝灰岩と重なりあう凝灰質砂岩は, 純粋な凝灰岩と通常の砂岩との中間的なモード組成を示す。しかも脆弱な軽石や火山ガラスも多く含むことから, 凝灰質砂岩の形成には, 酸性凝灰岩の淘汰・分級とともに, 非火山性砕屑物との混合が大きな役割を果たしたと推定される。竜神累層には凝灰質砂岩がよく発達しており, 竜神累層の粗粒堆積物の形成には, 堆積と同時期の火山噴出物からの寄与が大きかったものと考えられる。

The Shimanto Supergroup in the Kii Peninsula is divided into three groups, namely the Cretaceous Hidakagawa Group, the Eocene Otonashigawa Group and the Eocene to early Miocene Muro Group from north to south. The Ryujin Formation belongs to the Hidakagawa Group and its geological age is Campanian〜Maastrichtian. The Ryujin Formation is composed mainly of mudstones, associated with sandstones, tuffaceous sandstones, alternating sandstone and mudstone, green rocks, and acidic tuffs. The acidic tuffs are mostly thinly bedded, however there are some times thick-bedded, coarse-grained tuffs. At Mt. Chibiki in the southeast part of Gobo City, pumiceous tuffs, pumiceous green tuffs, white glassy tuffs and tuffaceous sandstones form a large sedimentary body, more than 300 m in thickness. Tuffaceous sandstones seem to be a mixture of sorted pyroclastic materials and non-volcanic clastics, the former, however, are more abundant than the later. There are abundant tuffaceous sandstones in the Ryujin Foramtion, which were formerly regarded as sandstone. Therefore, it is concluded that the acidic volcanic activity coeval with sedimentation, played a large role as a source for the coarse clastic sediments of the Ryujin Formation.