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

本研究的目的是在放射虫研究的基础上，重建本区早中三叠世的海洋环境。首先，在日本中部米诺地体早中三叠世硅质层序的连续地层剖面上进行了野外工作和岩石采样。其次，研究了早中三叠世放射虫组合的动物群组成，并与二叠纪晚期和侏罗纪放射虫组合进行了对比，描述了硅质岩的岩石学特征。下三叠统主要由硅质粘土岩和薄层黑色泥岩组成，放射虫遗迹稀少，种类很少。中三叠统主要为红层硅质岩序列，其中含有丰富的中生代放射虫，其种类向上变化较大。下三叠统硅质粘土岩逐渐演化为中三叠统层状硅质岩。另一方面，米诺地体上二叠统除碳酸盐岩相外，以层状硅质岩序列为代表，其中含有变化较大的古生代放射虫。在上述结果的基础上，认为早三叠世低纬潘地拉萨地区的海洋环境受到世界性缺氧事件的影响，下三叠统没有层状硅质岩是由于放射虫生产力低所致。从早三叠世硅质粘土岩到中三叠世层状硅质岩的岩性变化表明，放射虫在中三叠世恢复了高产。中三叠世红层硅质岩被认为是在含氧环境下形成的。认为晚二叠世至中三叠世放射虫的大规模动物群变化受控于海洋环境的变化。

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.