中央北海道南部地域における白亜系蝦夷累層群砂岩の化学組成  [in Japanese] Chemical composition of the sandstones from the Cretaceous Yezo Supergroup, south Central Hokkaido, Japan  [in Japanese]

白亜紀蝦夷累層群砂岩の全岩化学組成を, 北海道中軸部の南部地域で検討した.砂岩は, 高いSiO_2(平均76.4 wt%)量を示し, 君波ダイアグラムでは陸弧・開析された火山弧の判別領域にプロットされる.蝦夷累層群の砂岩は化学組成から, 1)中部蝦夷層群, 2)"東帯", 3)上部・下部蝦夷層群, にグループ分けができる.これは判別分析によっても確認された.砂岩化学組成における層間・層内偏差を中部蝦夷層群の二つの露頭で検証した.層間偏差は層内偏差より幅が大きい.これらの偏差は, タービダイトの堆積過程における粒径分化か, あるいは定着後の続成過程で形成された可能性が考えられる.これらの偏差の要因を推定するために, 仮想砂岩に対する特定の鉱物の付加・除去シミュレーションを行なった.その結果, 緑泥石のような粘土鉱物の付加とアルバイトの除去が偏差の主要な要因と考えられる.

Bulk rock chemical compositions of sandstones from the Cretaceous Yezo Supergroup in the south central part of Hokkaido are characteristically SiO_2-rich (76.4 wt% average). The sandstones are classified as of continental and/or dissected arc provenance on the Kiminami Diagram. Compositional grouping of the sandstones is clearly recognized as 1) Middle Yezo Group, 2) "Eastern Belt", and 3) Lower and Upper Yezo Groups. This grouping is also confirmed by discriminant analysis.Inter-stratal (bed-by-bed) and intra-stratal deviations in the sandstone chemistry were also examined at two selected outcrops in the Middle Yezo Group. Inter-stratal deviation is wider than intra-stratal deviation. This phenomenon is probably due to clast-size differentiation in turbiditic sedimentary process, or post-depositional diagenetic effect. No systematic solution, however, has yet been determined. Addition and removal of specific minerals to the composition of "virtual" sandstone were simulated in order to estimate the cause of the deviation. As a result, addition of clay minerals as chlorite and removal of albite are considered to be the main cause of the compositional deviation.