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In the northeastern Shimokita Peninsula, Aomori Prefecture, a Jurassic accretionary complex (AC) belonging to the North Kitakami Belt is distributed in three hilly areas of the Kuwabatayama, Katasakiyama and Omori. Although the AC in the Kuwabatayama area has been extensively studied and subdivided into the Late Jurassic Iwaya Unit and the Early Cretaceous Shitsukari Unit, those in the other areas are not fully understood except for a recent report of the detrital zircon U–Pb age from sandstone in the Omori area. We extracted radiolarian fossils such as Eucyrtidiellum cf. pyramis indicating the Late Jurassic (probably Kimmeridgian) from mudstone near the U–Pb-dated sandstone in the Omori area. Because this radiolarian age is close to the zircon U–Pb age, and the horizons of the mudstone and sandstone are close without any recognizable distinctive tectonic discontinuity between them, the clastic rocks in the Omori area may be stratigraphically continuous or contemporaneous sequences deposited around the Kimmeridgian. The ACs in the Katasakiyama and Omori areas are correlative to the Iwaya Unit in the Kuwabatayama area based on the lithology, geologic structure and clastic rock age. Therefore, the ACs in the northeastern Shimokita Peninsula can be divided into the Late Jurassic and the Early Cretaceous units; the former is the Iwaya Unit in the Kuwabatayama area and the unnamed ACs in the Omori and Katasakiyama areas, while the latter is the Shitsukari Unit in the Kuwabatayama area.

chishitsu chousa kenkyu houkoku/chishitsu chousa kenkyuu houkoku

Late Jurassic radiolarians from mudstone near the U–Pb-dated sandstone ofthe North Kitakami Belt in the northeastern Shimokita Peninsula, Tohoku, Japan