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. To clarify a possible décollement zone in Permian sequence of the mid‐Mesozoic accretionary complexes, we surveyed the Ryokami‐yama and Ryokami Formations and the Ryokami‐yama Thrust in the southern Chichibu terrane, eastern Mt. Ryokami, Kanto Mountains, Japan. We revealed that 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. Together with compilation of previous studies, we hypothesize that basaltic rocks underlying the Sakmarian chert acted as a décollement zone in the Permian sequences of some geologic units of the mid‐Mesozoic accretionary complexes. 西南日本の中期中生代付加体中のペルム系遠洋成シーケンス中におけるデコルマ面に着目し, 関東山地両神山の両神山スラストの構造的上位に位置する両神山チャート層の検討を行った. その結果, 同層のペルム系遠洋成シーケンスの岩相及び放散虫年代を明らかにした. 両神山チャート層の基底部は, 淡緑色の玄武岩凝灰角礫岩とその上位の赤レンガ色層厚チャートからなる. 赤レンガ色層厚チャートはドロマイトを挟み, ペルム系前期サクマーリアン期Pseudoalbaillella lomentaria帯の放散虫を産出する. さらに, 西南日本の中期中生代付加体に含まれるペルム系チャートに関する先行研究のコンパイルからは, 最も古い放散虫化石帯がPseudoalbaillella lomentaria帯にあたる地質体が多いことが明らかになった. ペルム系下部サクマーリアン期の