四国西部白亜系の砂岩組成と堆積環境  [in Japanese] Sandstone composition and sedimentary environments of the Cretaceous System in western Shikoku  [in Japanese]

四国西部白亜系の砂岩111試料について, その鉱物モード組成を求め, またその10試料について全岩分析を行い, それらの結果をDickinson et al.(1983)のQ-F-L図, Roser & Korsch (1986)のSiO_2/Al_2O_3-K_20/Na_20図を適用して, これらの堆積環境の検討を行なった。秩父累帯に分布する物部川層群(二及層・高島層・真穴層)と南海層群(菊野谷累層)はいずれもアプチアンに属すると考えられる。菊野谷累層・二及層・真穴層の砂岩は供給源地を大陸地塊とするが, 高島層のそれは大陸地塊と火山島弧とする。下部四万十超層群については, その5セクションについて比較検討した。コニアシアン〜サントニアンの砂岩の砕屑物は再輪廻造山帯より海底扇状地または深海平原で堆積したものであり, コニアシアンを含むそれ以前の砂岩は一部大陸地塊と火山島弧を後背地とし前島弧海盆で形成されたものである, と推定可能である。

Point-count data of a total of 111 samples from the Cretaceous sandstones in western Shikokua represented and the data are interpreted using the Q-F-L provenance-discrimination diagram proposed by Dickinson et al. (1983) In addition, chemical analyses for 10 sandstones from the Lower Shimanto Supergroup were carried out, and the results are discussed according to the SiO_2/Al_2O_3-K_20/Na_20 tectonic setting discrimination diagram proposed by Roser & Korsch (1986). In the Chichibu Belt, Aptian strata in the Northern Subbelt are represented by the Monobegawa Group, which consist of the Nigyu, Takashima and Maana Formations, and those in the Southern Subbelt are represented by the Nankai Group (Kikunotani Formation). According to the Q-F-L diagrm, 19 samples from the four formations mentioned aboveindicate two provenance fields. Sandstones from the Kikunotani, Nigyuand Maana Formations fall in the continental block field, whereas those from the Takashima Formation fall in both the continental block and the magmatic arc fields. The Cretaceous strata in the Shimanto Belt are represented by the Lower Shimanto Supergroup. The Q-F-L diagram for sandstones from the Yoshida, Kona, Takagushi, Kanayama-A and Ooura-A Sections in the Shimanto Belt indicates that detrital material was derived from a magmatic arc (dissected arc). Sandstones from the Kanayama-B and Ooura-B Sections fall in a field indicating their derivation from a recycled orogen. Whole-rock chemical data of sandstones from the Kanayama and Ooura Sections of the Coniasian to Santonian age fall in the PM (passive continental margin) field, and sandstones from the Yoshida, Kona and Takagushi Sections fall in or near the ACM (active continental margin) field (Fig. 4).