Permian brachiopods from boulders in the Pliocene basal conglomerate of Hitachi, central Japan, and their tectonic implications

Abstract

A Permian brachiopod fauna (the Ishinazaka Fauna), from shale and argillaceous limestone boulders in the basal conglomerate of the Pliocene Kume Formation of Hitachi, central Japan, is here described, and the age and palaeobiogeography of the fauna are discussed. The fossil-bearing boulders are probably derived from the Permian Ayukawa Formation belonging to the Hitachi Palaeozoic rocks. Therefore, the age and palaeobiogeography of the Ishinazaka Fauna suggest age and sedimentary site of the Ayukawa Formation, respectively. The faunal composition of the Ishinazaka Fauna, which consists of 17 species in 16 genera, resembles that of the middle Permian brachiopod fauna of the South Kitakami Belt, northeastern Japan, and thus indicates a middle Permian (Wordian) age. In terms of palaeobiogeography, the Ishinazaka Fauna, which includes both Boreal (antitropical) and Tethyan (tropical) elements, belongs to the Sino-Mongolian-Japanese Province which developed around North China (Sino-Korea) during the Wordian. The Hitachi area, during the Wordian, was probably part of a continental shelf bordering the eastern margin of North China. The Hitachi Palaeozoic rocks, especially the Permian Ayukawa Formation and the underlying Carboniferous Daioin Formation, are considered to be a southern extension of the Palaeozoic rocks of the South Kitakami Belt.