Change of marine environment based on analysis of Early-middle Triassic radiolarians

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This study aimed to reconstruct the Early-Middle Triassic marine environment of the Panthalassa region on the basis of radiolarian research. Firstly, field work and rock sampling were carried out in the continuous stratigraphic sections of the lower to Middle Triassic siliceous sequence of the Mino Terrane, central Japan. Secondly, the faunal composition of the Early-Middle Triassic radiolarian assemblages was examined and compared with the Late Permain and Jurassic radiolarian assemblages, and petrographical features of siliceous rocks were described.The lower Triassic is mainly composed of siliceous claystone with thin black mudstone layrs in which radiolarian remains are rare and their variety is very low. The Middle Triassic mainly consists of red bedded chert sequence in which Mesozoic type radiolarians are abundantly contained and their variety becomes higher upward. The lower Triassic siliceous claystone gradually changes to the Middle Triassic bedded chert.On the other hand, the Upper Permian of the Mino Terrane excluding carbonate facies is represented by bedded chert sequence in which the highly varied Paleozoic type radiolarians are contained.On the basis of above mentioned results, it is considered that the marine environment of the low-latitudinal Panthalassa region during Early Triassic time was affected by the worldwide anoxic event, and that no bedded chert of the Lower Triassic depended on the low productivity of radiolarians. The lithologic change from the Early Triassic siliceous claystone to the Middle Triassic bedded chert shows that the high productivity of radiolarians recovered during Middle Triassic time. The Middle Triassic red bedded chert is considered to be formed under the oxic environment. It is concluded that the large scaled faunal change of radiolarians through Late Permian to Middle Triassic time was controlled by the marine environmental change.