鳥取層群泥質岩相の堆積環境 : 普含寺泥岩層の層相変化について  [in Japanese] Sedimentary environments of muddy facies, the Middle Miocene Tottori Group : The facies change of the Fuganji Mudstone Member  [in Japanese]

中新統鳥取層群の泥質岩相である普含寺泥岩層の層相変化と化石から, 堆積環境と古環境の変遷について推定した。普含寺泥岩層は一連の堆積層で下部から上部に縞状頁岩相, 泥岩・シルト岩相および黒色泥岩相に岩相変化する。縞状頁岩相は剥離性の強い縞状の頁岩層と平行ラミナの発達したシルト〜砂岩層からなる。下部は無化石であるが上部では台島型の植物化石を産出する。しかし, 海生化石は伴わない。堆積環境は湖沼ないしは淡水の低湿地と考えられる。泥岩・シルト岩相は泥岩とシルト岩の互層〜細粒砂岩とシルト岩の互層である。縞状頁岩の上にかさなり, 陸生の植物化石のほかに海生の軟体動物化石が産出する。ニシン目・サケ目などの硬骨魚類化石を多産する。堆積環境は汽水〜浅海域が考えられる。黒色泥岩相は緻密・塊状の黒色泥岩で普含寺泥岩層の中〜上部を構成する。下部にはAcesta, Propeammusiumなどの海生軟体動物化石のほかにタラ目などの深海性魚類の耳石化石が産出するが, 上部では大型の化石は産出しない。堆積環境は陸棚下部〜半深海域と考えられる。鳥取層群の泥質岩相および産出化石の層相変化は中新世における堆積盆地の急速な沈降を示している。

The paleoenvironment of the Fuganji Mudstone Member (Tottori Group) is discussed based on its rock facies and biofacies. The Tottori Group ranges from Early to Middle Miocene and is more than 500 m in total thickness. The Fuganji Mudstone Member is divided into three types of sedimentary facies: that is banded shale facies, interbedded mudstone and siltstones facies and massive black mudstone facies in ascending order. The banded shale facies is developed chiefly in the lower part of the Member. It consists of banded shale, banded sandstone, and laminated mudstones and siltstones. Many land plant remains of the Daijima flora are yielded at Okamasu, Fuganji and some other places, though no marine fossils are found there. The lithofacies and fossil evidence indicates that this faciesis of lacustrine origin. The middle part of the Fuganji Mudstone Member consists chiefly of interbedded mudstone and silt-stone facies, with intercalation of acidic tuff, which conformably overlie the lower banded shale. These sedimentary rocks yield marine fishes such as Clupeiformes and Salmoniformes with a few of marine molluscan fossils in addition to the plants mentioned above. The sedimentary environment of this faciesis considered to be a brackish water lake or shallow sea. The black mudstone facies is common in the upper part of the Fuganji Mudstone Member. It consists of massive black mudstones with occasional acidic tuffs and turbidites. Marine molluscs such as Acesta and Propeammusium are fairly common in its lower half, but no fossils have been found in its upper half. The associated bathyal fossils such as otolith of a deep sea fish, benthic crustacean Bathynomus and so forth indicate that this facies was deposited in a bathyal environment. A thick pile of clastic sediments of the Fuganji Mudstone Member may have been accumulated rapidly at Early to Middle Miocene time.