Geochemical characteristics of organic black mudstones related to the Permian/Triassic boundary in pelagic sediments of Japan

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  • 西南日本の遠洋性堆積物にふくまれるペルム/トリアス境界に関連して形成された有機質黒色泥岩の地球化学的検討
  • Geochemical characteristics of organic

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Organic black mudstones and siliceous shales form marker beds of the Earliest Triassic in the bedded chert sequence of Southwest Japan. The organic black mudstones contain a large amount of organic carbon compared with usual bituminous shales. Geochemical analyses were made using X-ray fluorescence and neutron activation methods. This study aims at evaluation of provenance change in relation to accumulation of organic materials in the pelagic environment. The results can be summarized as follows: (1) Organic black mudstones and siliceous shales commonly show enrichment in K_2O, Rb, Cs, Hf, Nb and Th compared to PAAS (post Archean Australian shales). C_s/Rb ratio also shows that organic black mudstones and siliceous shales show similar composition to that of PAAS. (2) TiO_2/SiO_2 ratios in organic black mudstones, are commonly greater than those of siliceous shales. Cr, Y and Zr enrichment coincides with heavy REE (Rare Earth Elements) enrichment in some organic black mudstones indicating possible derivation from basic materials. (3) REE patterns of organic black mudstones exhibit negative Eu anomalies in some cases, but their values are rather smaller than those of silicic volcaniclastics. LREE/HREE ratios vary in vertical section, and obvious middle and heavy REE enrichments appear, due to presence of intermediate to basic volcanic material (4) OIT (oceanic island tholeiite) normalized spider diagram for organic black mudstones and siliceous shales shows similar pattern to those of CFB (continental flood basalt) and PAAS. (5) Depletion in CaO, MnO and Co is a common feature of organic black mudstones and siliceous shales, and is compatible with deposition below the CCD under anoxic conditions. (6) Uranium enrichment in organic black mudstones occurs, due to deposition under anoxic conditions. Although the results indicate local differences in the terrane provenance, organic black mudstones, in general, have an influence of intermediate to basic volcanic materials. Organic black mudstones are usually regarded as products of oceanic stagnation. But in pelagic conditions, organic material was formed by high primary productivity of marine planktons resulting from active upwelling of bottom currents. This points to a synchronous event of activated oceanic conditions in the earliest Griesbachian rapid sea-level rise.

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