福井県坂井郡三国町に分布する米脇層（14.5 ～ 12.5 Ma）は主に下部は堆積岩類から上部は火山岩類からなる．今回の調査では下部の堆積岩を苗代田凝灰質砂岩・泥岩部層，福良浜礫岩部層，安島南凝灰質角礫岩部層，米ヶ脇凝灰質角礫岩部層，宿凝灰岩部層に細分しその古環境の復元を行った．苗代田部層は化石や堆積相から下部外浜～内側陸棚で堆積した．また，コンボルート層理や異常堆積，層内断層など地層が固結する前に変形している．これは火山活動や地震が頻発した環境で堆積したことを示している．福良浜部層は苗代田部層を大規模なチャネルで削り込み堆積した主に河川成の堆積物で南東から北西に向かって流れる網状河川の堆積相で一部は扇状地三角州を形成していた．安島南部層は土石流堆積物を主とする扇状地性の堆積物で，米ヶ脇周辺では水中土石流堆積物である米ヶ脇部層が堆積した．宿部層はデイサイト～安山岩質の火砕流堆積物と凝灰岩からなり，安山岩火山弾を含む．堆積相は全体としては下位から内側陸棚→下部外浜→河川→扇状地と上方に浅海化する堆積環境の変化を示している．このような古環境の変化は日本海拡大終了後，引っ張り応力場から圧縮応力場に変わったことが原因と考えられる．また，苗代田部層は福浦ハイアロクラスタイトなどのデイサイト～安山岩質の海底火山活動が推定される．福良浜部層中に含まれる黒曜岩や真珠岩の礫は，流紋岩質の水中火山が近くにあったことを示唆する．安島南部層の土石流堆積物中には柱状節理の形状を残す安山岩角礫が見られ，安山岩の山体が近くにあった．そのほか，苗代田部層最上部では玄武岩類の活動も見られる．宿部層には径20 cm の火山弾が含まれ，安山岩主体の火山活動が数km 圏内で起こっていたと考えられる．これらのことから米脇層の下部堆積岩類の堆積時には流紋岩～玄武岩までのさまざまなタイプの火山活動が行われていた．The Komegawaki Formation (14.5-12.5 Ma) consists mainly of sedimentary rocks in the lower part and volcanic rocks in the upper part. The lower sedimentary rocks are subdivided into the Nawashiroda Tuffaceous Sandstone and Mudstone Member, the Fukurahama Conglomerate Member, the Anto-Minami Tuffaceous Conglomerate Member, the Komegawaki Tuffaceous Conglomerate Member, and the Shuku Tuff Member, in ascending order. The Nawashiroda Member was deposited in the lower shoreface to inner shelf based on fossils and sedimentary facies, but also contains submerged autoclastic lava in some parts. Convolute bedding, anomalous sedimentation, and intrabedded faults deformed the strata before the strata were consolidated. This indicates that the sediments were deposited in an environment of frequent volcanic activity and earthquakes. The Fukurahama Member is sedimentary facies within a reticulated river flowing from southeast to northwest. From the analysis of the conglomerate, it was mainly deposited by large-scale channel cutting of the Nawashiroda Member. The Anto-Minami Member consists mainly of debris-flow deposits. The Komegawaki Member consist mainly of debris-flow deposits in water. The Shuku Member is composed of dacitic to andesitic pyroclastic flow deposits and tuff that was secondarily deposited by hydrothermal processes, and includes andesite volcanic bombs. These paleoenvironmental changes are thought to be caused by a change from a tensile stress field to a compressive stress field after the end of the Japan Sea expansion.　In addition, lava, hyaloclastite, subaqueous debris flow deposits and subaqueous pyroclastic flow deposits were observed during deposition of the Nawashiroda Member, and pisolite and pumice tuff were observed in the Fukurahama Member. In particular, obsidian and pearlite gravels in the Fukurahama Member suggest the presence of a subaqueous volcano of rhyolite nearby. Blocks of andesite that retain the shape of columnar joints are found in the Anto-Minami Member debris flow deposits, suggesting that there was a mountain body of andesite nearby. The presence of a 20 cm diameter volcanic bomb suggests that andesite volcanism was taking place within a few kilometers of the Shuku Member at the time it was deposited.

source:http://recre.ou.u-fukui.ac.jp/research.html#kiyou