Mesozoic radiolarian fossils from mudstone within the accretionary complex in the southwestern margin of the North Kitakami Belt, eastern Morioka, Iwate Prefecture, Northeast Japan

UCHINO Takayuki, SUZUKI Noritoshi

doi:10.9795/bullgsj.72.119

<p>The Kitakami Massif in the Tohoku region, Northeast Japan, is composed largely of the South Kitakami Belt to the south and the North Kitakami Belt to the north. The North Kitakami Belt consists mainly of a Jurassic accretionary complex, but few fossils have been reported from the southwestern margin of the belt. In this study, radiolarian fossils were newly obtained from mudstone in the Sotoyama district in eastern Morioka. This mudstone occurs near the location of sandstone that appears to be the Early Jurassic according to U–Pb dating of detrital zircon. It is nearly impossible to specify its taxonomic name definitively due to the poorly-preserved radiolarians, but their assemblage certainly indicates the Mesozoic. Furthermore, circumstantial evidence of the common families and species inferred from shape suggests the possibility of the late Early–Middle Jurassic; the assemblage supports the Early Jurassic zircon age previously obtained from the neighboring sandstone. The radiolarian assemblage from the Sotoyama district is also younger than the Triassic radiolarians, in a similar state of preservation to the Sotoyama district, from the argillaceous rock of the North Kitakami Belt in the Hayachinesan district. This fact supports the existing idea that the accretionary complex in the Sotoyama district belongs to a different tectonostratigraphic unit from that in the Hayachinesan district.</p>

Mesozoic radiolarian fossils from mudstone within the accretionary complex in the southwestern margin of the North Kitakami Belt, eastern Morioka, Iwate Prefecture, Northeast Japan

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