北部北上帯「陸中関」地域から産出した後期ジュラ紀放散虫化石

抄録

Radiolarian fossils were discovered from a mudstone of the Seki Formation at the Rikuchu-Seki district in the North Kitakami Belt. The mudstone containing the radiolarians comprises a part of 'chert-clastics sequences' together with stratigraphically underlying cherts. The radiolarian fauna consists of several genera including Archaeodictyomitra, Parvicingula (?), Setho-capsa, Transhsuum, Xitus (?), Zhamoidellum and others. They are generally regarded as Jurassic radiolarians, but state of their preservation is not good enough to determine preciser age of this fauna. Nevertheless, the age is considered to be middle Late Jurassic (Kimmeridgian) on the basis of the presence of Transhsuum sp. cf. Tr. maxwelli (Pessagno), Sethocapsa hexagona Hori and Zhamoidellum ovum Dumitrica. The above proves that a Late Jurassic accretionary complex (Seki Formation) is widely exposed in this district.