论文部分内容阅读
A detailed study of the mineral composition and microstructure of the black shales associated with OAEla(Eastern Russian Platform),OAElb(Middle Caspian),and the host rocks has been carried out using X-ray diffraction,scanning electron microscopic,and microprobe analyses.The results provide important constraints for depositional environments in the sedimentary basins.Black shales with pyrite framboids imply euxinic(sulfidic)conditions with increased organic matter preservation.Disintegrating framboids suggest partial or complete dissolution of the organic matter inside the framboids due to increasing water oxygenation.OAEla on the Eastern Russian Platform is heterogeneous as it includes thin interbeds of concretionary coccolith limestones within the interval of bituminous shales,and correlates with the Lower Aptian Rhagodiscus angustus nannofossil zone.The coccolith limestones indicate short intermittent episodes of interrupted stagnation,rapid oxygenation,and restoration of normal marine conditions.The presence of montmorillonite,albite,microcline,and diopside in the bituminous siltstones and in the host siltstones of OAEla on the Eastern Russian Platform,as well as a high content of titanium in the black siltstones correlated with OAElb in the Middle Caspian allow proposing significant input of pyroclastic material into the extant sea.
A detailed study of the mineral composition and microstructure of the black shales associated with OAEla (Eastern Russian Platform), OAElb (Middle Caspian), and the host rocks has been carried out using X-ray diffraction, scanning electron microscopic, and microprobe analyzes. The results provide important constraints for depositional environments in the sedimentary basins. Black shales with pyrite framboids imply euxinic (sulfidic) conditions with increased organic matter preservation. Disintegrating framboids suggest partial or complete dissolution of the organic matter inside the framboids due to increasing water oxygenation. OAEla on the Eastern Russian Platform is heterogeneous as it includes thin interbeds of concretionary coccolith limestones within the interval of bituminous shales, and correlates with the Lower Aptian Rhagodiscus angusus nannofossil zone. Coccolith limestones indicate short intermittent episodes of interrupted stagnation, rapid oxygenation, and restoration of normal mari ne conditions. presence of montmorillonite, albite, microcline, and diopside in the bituminous siltstones and in the host siltstones of OAEla on the Eastern Russian Platform, as well as a high content of titanium in the black siltstones correlated with OAElb in the Middle Caspian allow proposing significant input of pyroclastic material into the extant sea.