1987 年 42 巻 p. 7-13
Fossil evidence of radiolarians reveals the age of the Shiriya Group (olistostrome facies) in the North Kitakami Belt (s. l.). Some olistoliths of chert are Early Jurassic and Middle-Late Jurassic in age. A tuffaceous mudstone sample contains the following radiolarians; Pantanellium lanceola (Parona), Parvicingula boesii (Parona), Pseudodictyomitra sp. cf. P. carpatica (Lozyniak) and others. The association of the radiolarians indicates that the examined sample is late Tithonian-earliest Cretaceous in age. It is not clear whether the tuffaceous mudstone represents a matrix of the olistostrome or it is also an olistolith itself within the olistostrome. It is certain, however, that the formation of the olistostrome continued during late Tithonian-earliest Cretaceous or later in the North Kitakami Belt (s. l.).