Geotectonic setting of the Upper Permian Mizukoshi Formation, central Kyushu, southwest Japan

Abstract

The Upper Permian (Lopingian) Mizukoshi Formation, distributed throughout the Mizukoshi area of central Kyushu, southwest Japan, is composed chiefly of black shale along with intercalated sandstone and conglomerate (Usuginu-type conglomerate), making a total thickness of 1,690 m. The Mizukoshi Formation is lithologically similar to the Toyoma Formation in the South Kitakami Belt, the upper Moribu Formation in the Hida Gaien Belt, and the Kuma Formation in the Kurosegawa Belt. The fusulinoidean fauna from limestone clasts within conglomerates in the upper part of the Mizukoshi Formation is characterized by the presence of Lepidolina species and the absence of Neoschwagerina and Yabeina species. In terms of generic composition, the fusulinoidean fauna of the Mizukoshi Formation shows a close affinity to those of the South Kitakami and Hida Gaien Belts, but differs from that of the Kurosegawa Belt. The brachiopod fauna from sandy shale and calcareous sandstone in the upper part of the Mizukoshi Formation, consisting of 22 species in 19 genera, is a Boreal- Tethyan mixed fauna, although the Boreal types are predominant. The Mizukoshi fauna is allied with the Middle to Late Permian brachiopod faunas of the Hida Gaien and South Kitakami Belts, the fauna of South Primorye, eastern Russia, and especially with the fauna of the Moribu area in the Hida Gaien Belt, central Japan. The above data strongly suggest that the Mizukoshi Formation represents the SW extension of the Permian rocks of the Hida Gaien Belt, thereby placing the formation within the South Kitakami Terrane. The dominantly Boreal-type assemblage within the Mizukoshi fauna indicates that during the Middle- Late Permian the Mizukoshi area was probably located between the Hida Gaien Belt to the north and the South Kitakami Belt to the south, bordering the eastern margin of North China. This conclusion supports the strike-slip model that describes large-scale sinistral strike-slip movement (approximately 1,500-2,000 km of displacement) along the Tanakura Tectonic Line (TTL) -Median Tectonic Line (MTL) from the Early Cretaceous to the Palaeogene.