Stratigraphy and geologic development of the Carboniferous to Permian strata in the Atetsu region, Akiyoshi Terrane, Southwest Japan

Stratigraphies and developmental history of the Carboniferous to Permian strata of the Akiyoshi Terrane were reconstructed on the basis of detailed investigations of the radiolarian biostratigraphy and geologic structure from the view point of an accretional active margin setting.
The Akiyoshi Terrane is characterized by major two different facies; carbonate, and siliceous and terrigenous rock facies. The carbonate rocks in the Atetsu region are represented by a reef complex, the Atetsu Limestone. The siliceous and terrigenous rocks are grouped into three facies types; the Terauchi, Fuyori, and Taniai Types, due to their stratigraphic and structural characteristics. The Terauchi Type is characterized by a coherent sequence, encloses remarkable amount of limestone clasts, and conformably overlies the Atetsu Limestone. This Type is trench-fill sediments covering directly the colliding seamount. The Fuyori Type is composed of the lower chert, the middle siliceous mudstone, and the upper coarser elastic rocks. The Fuyori Type is corresponds to the oceanic plate stratigraphy indicating the landward migration of the depositional site. The Taniai Type is a mélange suite containing various-sized exotic blocks of different lithologies.
Investigations of geologic structures, especially recognition of the nappe boundary bending folds, indicates that the Carboniferous to Permian rocks in the Atetsu region formed the major two nappes that were probably formed by the collision of seamount.
The biostratigraphic study concentrates to the correlation between radiolarian and fusulinid assemblages. The first occurrence of F. sp. cf. F. japonicas (radiolaria) is correlated to the base of Lepidolina imanntrai Zone (fusulinacean zone), and indicates the Middle / Late Permian boundary.
The age of the lower limit of the Terauchi Type sediments indicates the arrival time of the seamounts to a trench. The seamount capped by the Atetsu Limestone collided during the F. sp. cf. F. japonicas Zone (Late Permian). The arrival times of several seamounts in the Akiyoshi Terrane are nearly same among the different regions. This indicates that the seamounts in the Akiyoshi Terrane were arranged nearly parallel to the trench.