粘土鉱物から見た古環境の解析 : 来馬層群の堆積環境

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Clay mineral assemblage reflects sedimentary environment such as difference of source materials, sedimentary temperature, and depth of deposition. Sequential clay mineralogical analyses of fossil free stratum give constraint on depositional environment of sedimentary rocks. The clay mineralogical analyses of the Kuruma Group, early Jurassic thick clastic sediments for stratum by stratum method made clear frequent transition of sedimentary environment. The clay mineral composition of the Kuruma Group is characterized by fluctuation of kaolinite ratio to smectite and mica clay minerals. Increase of kaolinite content suggests shallow and near-coast sedimentary environment influenced in freshwater. The presence of brackish molluscan fossils and fossilized trunk also supports near coast environment. Clay mineralogical analyses suggest that increase of smectite and mica clay minerals contents reflect off shore sedimentary environment. Marine molluscan such as ammonites, crinoids and brachiopods support this effect. The Kuruma Group is situated on the Hida marginal Terrane, characterized by Permian to Triassic accretion sediments of serpentinite melange with the Hida semi-continental margin. Chemical compositions of this Group indicate that the source of clastic sediments of the Kuruma Group is acidic rocks rather than basic rocks composed of the Hida marginal Terrane. Dramatic change of kaolinite and smectite ratios of the Kuruma Group suggests g1obal oceanic environmental change such as fluctuation of sea level due to subdivision of super-continent (Pangea). Decrease of kaolinite and increase of smectite suggest the rise of sea level. It is plausible that global oceanic event gave influence on the near coast sediments such as the Kuruma Group. The studies of sequence stratigraphy, isotopic data and paleontological research also support the presence of global oceanic events. The clay minerals have recorded the change of sedimentary system and the geological phenomena caused by various causes. Clay mineral analysis is possible to contribute to the studies on the paleo-environmental analysis, because the clay minerals are extremely sensitive to change of the environments. This study reports new paleo-environmental analysis of the early Jurassic Kuruma Group based on the analyses of clay mineral composition and the chemical composition of the sedimentary rocks related with the change of paleontological faces and global oceanic events.
日本粘土学会の論文
1998-12-28

粘土鉱物から見た古環境の解析 : 来馬層群の堆積環境

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