Triassic radiolarians from chert of the Chichibu Belt in the Toyohashi district,Aichi Prefecture, Southwest Japan.

Abstract

Accretionary complex of the Chichibu Belt in the Toyohashi district, Aichi Prefecture, Southwest Japan is tectonostratigraphically divided into Unit A, B and C by Niwa and Otsuka (2001). Based on the radiolarian biostratigraphy, the oceanic plate stratigraphy (OPS) of Unit A and Unit B (Niwa and Otsuka, 2001) is reconstructed. The OPS of Unit A is composed of Lower Permian to upper Middle Jurassic chert, upper Middle Jurassic siliceous mudstone and upper Middle to lower Upper Jurassic mudstone, in ascending order. The OPS of Unit B consists of Lower Permian to lower Middle Jurassic chert, upper Lower to upper Middle Jurassic siliceous mudstone and middle Lower to lower Upper Jurassic mudstone, in ascending order. The duration of the deposition of mudstone of Unit B is much longer than that of the other tectonostratigraphic units of the Jurassic accretionary complexes in Japan. Furthermore, in Unit B, there is a mudstone deposited during Pliensbachian age which is older than the age of the boundary of chert and siliceous mudstone. These suggest that not only Unit B can be subdivided chronologically, but also the reexamination of the tectonostratigraphic division by Niwa and Otsuka (2001) is required.