Abstract
Latest Jurassic to Cretaceous radiolarians have been identified in coarse-grained sandstones from the Middle Miocene Chichibumachi Formation of the Akahira Group, which occurs in the Chichibu Basin, Saitama Prefecture, central Japan. The following radiolarians were identified: Pseudodictyomitra lanceloti Schaaf, Holocryptocanium barbui Dumitrica, Diacanthocapsa betica O'Dogherty, Thanarla brouweri (Tan), Archaeodictyomitra sp. A, Archaeodictyomitra sp. B, Archaeodictyomitra? sp., Sethocapsa sp. A, Sethocapsa sp. B, Sethocapsa? sp., and Diacanthocapsa? sp. Radiolarian-bearing sandstone is present at ca. 22 m above the level containing Middle Miocene Paleoparadoxia tabatai, which is a marine mammalian fossil. Radiolarians occur within the sandstone as isolated specimens, along with fragments of bivalves, foraminifers, plant fossils, and quartz grains, some of the radiolarian specimens appear to have been reworked from the Ishido Formation or younger formations of the Sanchu Cretaceous System that crops out to the northwest of the Chichibu Basin.
