Permian Nishiki Group in the Muikaichi-cho, western part of the Sangun-Chugoku Belt, Shimane Prefecture. Southwest Japan

Abstract

The middle to upper Permian Nishiki Group distributed in the Muikaichi-cho area is divided lithologically into Na (chert facies), Nb (mudstone facies), Nc (sandstone facies) formations in ascending order. Na formation, 120m in maximum thickness is composed of massive and bedded cherts which are intercalated with thin reddish mudstone layers and lenticular bodies of fine sandstone. Nb formation, 200m in maximum thickness, is mainly composed of black mudstone which is frequently intercalated with acidic tuffs. Nc formation is 500-1000m in thickness, and consists mostly of massive sandstone. The newly obtained Permian radiolarians have ascertained the hitherto undetermined age of the Nishiki group. Na formation includes upper part of Albaillella sinuata Range-zone to Pseudoalbaillella sp. C Assemblage-zone, and Nb formation includes Ps. sp. C A-zone to lower part of Follicucullus scholasticus A-zone. Nc formation also includes lower part of Fo. scholasticus A-zone. Then Nishiki Group of the area is safely correlated to Leonardian to Guadalupian (middle to late Permian) in age. The Permian radiolarian biostratigraphy has been established in the Tamba Belt, where the strata from upper part of A. sinuata R-zone to lower part of Fo. scholasticus A-zone are represented by bedded cherts of only 13 meters thick, while the same ones of the present area are expressed by clastic rocks of mudstone and sandstone of more than 500 meters thick, although the lowest part of which includes cherty rocks. The frequently intercalated acidic tuffs of the middle Permian, especially of Fo. monacanthus A-zone, indicates the existence of active and probably extensive acid volcanisms on the neighbouring hinterland.