火山活動と堆積作用からみた鳥取地域の前期中新世末〜中期中新世古地理  [in Japanese] Latest Early to Middle Miocene paleogeography of the Tottori area viewed from volcanism and sedimentation  [in Japanese]

鳥取地域の前期中新世末〜中期中新世火山活動, 堆積作用および古地理は次のようにまとめられる。小田安山岩層は模式地付近を中心に水中噴出した溶岩から構成され, 荒金火砕岩層は分布域北西方の噴火口より発生した大規模な水中火砕流からなる。これらの火山活動は, 比較的深い海域で起こった。円通寺礫岩砂岩層, 諸鹿礫岩層の平均礫径や礫組成の変化は, それらが南西〜南方の基盤から由来したことを表している。円通寺礫岩砂岩層は湖沼域から海域へ供給された扇状地性〜ファンデルタ性堆積物で, 諸鹿礫岩層は汽水〜浅海環境で堆積した。駟馳山砂岩泥岩層の砂岩は主として花崗岩からなる北方基盤から, 普含寺泥岩層の砂岩は多様な地質構成をもつ南方基盤から由来した。前期中新世末〜中期中新世の鳥取地域とその周辺には, 駟馳山砂岩泥岩層の源岩となった隆起帯と, 円通寺礫岩砂岩層, 諸鹿礫岩層と, 普含寺泥岩層の砂岩を供給した隆起帯が存在した。

The paleogeography of the Tottori area during latest Early to Middle Miocene is clarified based on the volcano stratigraphical and sedimentological studies of the Iwami Formation. The Oda Andesite Memberis composed mainly of andesite lava flows of subaqueous extrusion from the vent near Oda. The Aragane pyroclastic Member consists mainly of a large amount of submarine dacitic pyroclastic flows. Lateral changes in thickness and facies of the Aragane Member reveal that the pyroclastic flows were violently ejected from the vent situated in the northwest of the distributed area. The bimodal volcanism represented by the Oda and Aragane members took place under a relatively deep marine environment. Sedimentological studies indicate that the clasts in the Entsuji Conglomerate and Sandstone and Moroga Conglomerate members were supplied from basement distributed in the south to southeast area. The gravels of the Entsuji Member are represented by Neogene andesites and basalts in the lower part, and by Mesozoic cherts in the upper. This compositional change suggests that the southward transgression resulted in southward retreat of the source area. The Entsuji Member can be interpreted as alluvial fan or fan delta deposits. The Moroga Member accumulated under a brackish-water to shallow sea environment. Sandstone composition shows that the Shichiyama Sandstone and Mudstone Member were supplied from a northern source rock composed mainly of granite, and that the sandstones of the Fuganji Mudstone Member were derived from the various source rocks of the south. Two zones of uplift exsited in and around the Tottori area during latest Early to Middle Miocene time. One is the source area of the conglomerates of the Entsuji and Moroga members and the sandstones of the Fuganji Member, trending E-W between the San-in Green Tuff and First Setouchi provinces. This zone has intermittently been uplifted since Early Miocene. The other is the source area of sandstones of the Shichiyama Member, trending NE-SW from Tottori to the Hokuriku area.