Lithology and radiolarian age of the Ryokami‐yama Chert Formation in eastern Mt. Ryokami: Possible décollement zone in Permian pelagic sequence in mid‐Mesozoic accretionary complexes of Southwest Japan

Uppermost Permian–lowermost Triassic siliceous claystone is well‐known as being a possible décollement zone in accretionary processes of mid‐Mesozoic accretionary complexes in Southwest Japan; meanwhile, fewer studies have focused on other décollement zones in Permian pelagic sequences. The Ryokami‐yama Thrust in the Southern Chichibu terrane, eastern Mt. Ryokami, Kanto Mountains, has been considered an out‐of‐sequence thrust and might show a former décollement zone in the formation's Permian pelagic sequences. This study shows the Permian pelagic sequences (lithology and radiolarian age) of the Ryokami‐yama Chert Formation on the Ryokami‐yama Thrust. The basal part of the Ryokami‐yama Chert Formation consists of pale‐green basaltic tuff breccia and overlying red‐ocher thick‐bedded chert intercalating dolomites. The red‐ocher thick‐bedded chert yielded radiolarians of the Pseudoalbaillella lomentaria Range‐Zone of the Sakmarian (Cisuralian: lower Permian). Compilation of the Permian pelagic sequences of the mid‐Mesozoic accretionary complexes in previous studies indicates that the bases of the Permian chert of many geologic units are concentrated in the Pseudoalbaillella lomentaria Range‐Zone. The following hypothesis is suggested here: the basaltic rock underlying the Sakmarian chert acted as a décollement zone in the Permian pelagic sequences of some geologic units of the mid‐Mesozoic accretionary complexes.