堆積相と構造運動 : 北部フォッサマグナ頸城-小谷地域のタービダイト堆積盆  [in Japanese] Sedimentary facies and tectonic movement : Turbidite basin of the Kubiki-Otari district in the northern Fossa Magna  [in Japanese]

フォッサマグナ最北部に位置する新潟堆積盆南西部の新生界タービダイト層の層相と古地理について概説し, 堆積相と構造時階との関わりを検討した。中新世中期から後期にかけてと中新世末から鮮新世にかけての2層準にタービダイト層が発達する。前期のタービダイト層は典型的な海底扇状地様式をなして堆積したが, これは堆積盆が形成・深化する過程で形成されたものである。他方, 後期のタービダイト層は様々な堆積様式を取るが, 後の褶曲変形の著しい地域では, 堆積盆は地溝状をなし, 軸流による粗粒砕屑物の層相変化が特徴的であり, 圧縮場での褶曲の始まりが示唆される。

A thick Neogene sequence is distributed in the Kubiki-Otari district of the northernmost part of the Fossa Magna. The Neogene, mainly composed of terrestrial clastic rocks with submarine volcanic rocks, is deposited in the sedimentary basin formed with relation to the opening of Japan Sea and the formation of the Fossa Magna. The turbidite sequences are observed at two stages; the one stage is at the Middle Miocene, and the other is at the Upper Miocene to Pliocene. The two types of depositional system are distinguished for these turbidite sequences, that is, the submarine fan turbidite and the confined trough-filled turbidite. The former sedimentary facies is characterized by the regular vertical change such as fining-upward and coarsening-upward sequences, and by the rapid lateral variation from the sandstone-rich bodies to the predominant mudstone bodies. On the other hand, the later sedimentary facies isn't basically characterized by the lateral change, but by the temporal variation between sandstone-rich and mudstone-rich facies. Furthernmore, the temporal and spatial distribution of sedimentary facies in the later system shows clearly the existence of tectonic highs surrounding the basin. The difference of the depositional system is considered to have resulted from the tectonic background at the depositional time, that is, the submarine fan system developes under the tension or neutral stress. On the other hand, the confined trough-fined system is considered to be formed under the compression stress.