Jurassic Radiolarians and Other Microfauna Recovered from the Trace Fossils of the Kiritani Formation of the Tetori Group in the Yatsuo Area, Toyama Prefecture, Northern Central Japan

Abstract

Age-significant Jurassic radiolarians and other microfossils are recognized for the first time from the Kiritani Formation of the Higashisakamori Subgroup of the Tetori Group, a Middle Jurassic to late Early Cretaceous group of shallow- to nonmarine sedimentary strata in northern Central Japan. The Kiritani Formation in the Jinzu Region, eastern part of the Tetori Basin, is mainly composed of marine terrigenous clastic rocks: conglomerate, sandstone, and sandy siltstone including unlined tubelike trace fossils. Radiolarians are present only in the trace fossils, not in the surrounding sandy siltstone. The radiolarian age of the Kiritani Formation based on the European radiolarian zonation is Callovian-early Tithonian by the occurrence of Zhamoidellum ovum. The Kiritani Formation can also be correlated with the Arimine Formation, another marine formation of the Jinzu Region, which yields radiolarians of the same age.Microfossils consist of various taxa: radiolarians, rhaxellid sponge spicules, prodissoconchs of juvenile bivalves, a charophyte gyrogonite, and a planktonic foraminifera. The radiolarian assemblages show a high ratio of spumellarians over nassellarians in the number of specimens. These microfossil assemblages suggest that the Kiritani Formation was deposited in a shallow marine environment.