Geochemical recognition criteria of pelagic and hemipelagic bedded red cherts of the Permian to Jurassic ages from Ie and Izena Islands, Okinawa, Japan

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Four sequences of bedded red cherts of the Permian to the Jurassic ages from Ie and Izena Islands were chosen and studied to investigate whether these sequences were formed in deep waters of pelagic environments. Among the four sequences, the bedded red chert sequence in Mt. Gusuku (Tacchu) shows true pelagic features where unicolored and thinly bedded red cherts of the Late Triassic to the Late Jurassic ages are exposed as thickly as about 70m. The other three sequences are slightly mixed with bedded gray cherts and do not show true pelagic facies. Two sequences were identified as turbidites. 64 rock samples were systematically sampled from these four sequences and geochemically analyzed for their major and trace element concentrations in the bulk fraction as well as in the carbonate fraction. Chemical stratigraphy of the red cherts in Tacchu indicated systematic downward increases of major and trace metallic elements, suggesting plate drifting toward the equator from the southern hemisphere during the Late Triassic through the Late Jurassic. The degree of correlation between Fe and Mg in the noncalcareous fraction or among other major elements indicates that the bedded red cherts in Tacchu were truely deposited in the pelagic deep-sea environments, whereas other three sequences studied in Ie and Izena Islands were deposited in hemipelagic environments.
日本堆積学会の論文