エンヨウセイ タイセキブツチュウ ノ ペルムキマツ カイヨウ ムサンソ ジヘン ト P/T キョウカイ
Late Permian Anoxic Event and P/T Boundary in Pelagic Sediments of Southwest Japan
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The P/T (Permian-Triassic) boundary has been recently recovered in siliceous rock sequences of Southwest Japan. The sequence representing oceanic plate stratigraphy, compris-es pelagic bedded cherts, embedded in the Jurassic accretionary complexes, and the boundary is marked by black carbonaceous mudstones indicating appearence of a drastic environemtnal change comparative to that of the K/T boundary in lithologic feature.
The sequence of Upper Permian bedded cherts is characterized by black or grey organic bedded cherts enriched by sulfide which gradually change from red bedded cherts of late Middle Permian age. The sequence of the P/T boundary and related rocks comprise:
1) grey to black Neoalbaillella radiolaria bearing bedded cherts, 2) grey siliceous claystones of possible boundary or Earliest Triassic, 3) black carbonaceous mudstones, and 4) Smithian (Early Triassic) conodont bearing grey siliceous claystones in ascending order. And at present apparent Griesbachian and Dienerian fossils have not been discriminated.
Occurrence of 1) black organic bedded cherts indicates the anoxia occurred already in the ocean, prior to the Latest Permian, i.e. the high sea level, warm oceans, high value of primary production. Concerning the size and structure of the Late Permian radiolarian skeletons, Neoalbaillella and some other polycystines are characterized by small, fine and complicated shaped of tropical features. Decreased amount of siliceous skeletons in 2) siliceous claystones indicates extinction of Permian polycystine radiolaria from latest Permian to earliest Triassic time.
The lithologic and radiolarian faunal variation in early Late Permian indicates shifting of sedimentary environment from oxic to oxygen depleted (or anoxic), which clearly coincides the timing of the volcanic activity of the continental flood basalt such as the Siberian Traps. And also this shifting is regarded to have occurred in middle Middle Permian, which is indicated by color change of bedded cherts from red to green and acid volcanic activity occurred in the surrounding terranes of intraplate oceanic arc setting. The enviornmental change of system and conditions between the greenhouse and icehouse preceeding to the last of the Paleozoic is regarded to be appeared in early Late Permian. The black carbonaceous mudstones inter-calated in siliceous claystones of the P/T boundary, although probably not representing true conditions, seem to be correlated with many mass extinction events.
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