17:15 〜 18:45
[BPT03-P02] Stratigraphic and geochemical information for the Upper Permian to Early Triassic Panthalassic pelagic carbonate sequence, Taho Limestone, Japan
キーワード：ペルム紀–三畳紀境界、田穂石灰岩
The Jurassic accretionary complex in Japan contains a Paleozoic to Mesozoic carbonate sequence deposited in the Panthalassa, which is suitable for reconstructing the paleoenvironment in pelagic Panthalassa. The Taho Limestone is one of the best-documented Japanese seamount carbonate succession covering from the Lopingian (upper Permian) to the Lower Triassic. A borehole core, the Taho Core, was drilled on the Taho Limestone in 1994, but previous paleontological and geochemical studies of Taho core have been limited to the short interval encompassing the Permian–Triassic boundary. Here, we have documented the detailed litho-, bio-, and chemostratigraphy of the Taho Core to shed light on the depositional environment from the Lopingian to the Lower Triassic.
The Taho Core consists of two formations: the Shirokawa and Taho formations in ascending order. The lower part of the Shirokawa Formation consists of an alternation of limestone and dolostone, which is overlain by a ~20 m thick dolomite layer. The stable carbon isotope ratios (δ13Ccarb) of the Shirokawa Formation range from ~3 to 4‰, typical values for the Lopingian Tethyan sedimentary record. The Shirokawa Formation is overlain by the Taho Formation and the boundary between these two formations is accompanied by the sharp negative carbon isotopic spike characteristic of the end-Permian mass extinction event. The Taho Formation of this core consists of gray limestone and dolomitic limestone intercalated with turbidite and limestone breccia. Judging from the frequently appearing breccia and turbidite, the Taho Limestone could have been deposited between the marginal basin and lower upper slope. Conodont occurrences and carbon isotopic stratigraphy show that the Taho Formation in this core covers the lowest Griesbachian (lower Indudan, Lower Triassic) to the base of Smithian (lower Olenekian, Lower Triassic) with some hiatuses. We also measured osmium isotope ratio (187Os/188Os) of the Taho Limestone. Age-corrected 187Os/188Os values range from ~0.37 to 0.65, consistent with previously reported Lopingian to Lower Triassic values.
The Taho Core consists of two formations: the Shirokawa and Taho formations in ascending order. The lower part of the Shirokawa Formation consists of an alternation of limestone and dolostone, which is overlain by a ~20 m thick dolomite layer. The stable carbon isotope ratios (δ13Ccarb) of the Shirokawa Formation range from ~3 to 4‰, typical values for the Lopingian Tethyan sedimentary record. The Shirokawa Formation is overlain by the Taho Formation and the boundary between these two formations is accompanied by the sharp negative carbon isotopic spike characteristic of the end-Permian mass extinction event. The Taho Formation of this core consists of gray limestone and dolomitic limestone intercalated with turbidite and limestone breccia. Judging from the frequently appearing breccia and turbidite, the Taho Limestone could have been deposited between the marginal basin and lower upper slope. Conodont occurrences and carbon isotopic stratigraphy show that the Taho Formation in this core covers the lowest Griesbachian (lower Indudan, Lower Triassic) to the base of Smithian (lower Olenekian, Lower Triassic) with some hiatuses. We also measured osmium isotope ratio (187Os/188Os) of the Taho Limestone. Age-corrected 187Os/188Os values range from ~0.37 to 0.65, consistent with previously reported Lopingian to Lower Triassic values.