Geology and accretionary age of the Otori Unit, North Kitakami Belt

Abstract

The North Kitakami Belt in Northeast Japan mainly comprises accretionary complexes formed during the Jurassic to earliest Cretaceous. The accretionary complex of the North Kitakami Belt is less studied compared to the age-equivalent accretionary complexes in Southwest Japan. Here, we provide additional data on the accretionary complex formerly classified as the Otori Unit, distributed in the northeastern part of the North Kitakami Belt in the upper reaches of the Akka River in Iwate Prefecture. Based on detailed field mapping, we clarified that the Otori Unit is composed of the structurally lower coherent facies of chert and siliceous mudstone (Okoshizawa Subunit) and the structurally upper mixed facies of mudstone, sandstone, chert and minor basaltic rocks (Osakamoto Subunit). Manganese nodules from siliceous mudstone within the Okoshizawa Subunit yielded radiolarians indicating the Bathonian (upper Middle Jurassic). Detrital zircon grains from sandstone in the Osakamoto Subunit has a youngest age of ~170 Ma (YC1σ: 171.8 ± 2.4 Ma; YSG: 170.9 ± 3.8 Ma). Based on the radiolarian and detrital zircon ages, the accretionary age of the Otori Unit is estimated as the Bathonian. Our new data were also considered to discuss the correlation between the North Kitakami Belt and the Southern Chichibu Belt in Southwest Japan. The Otori Unit corresponds to the Ohirayama Unit of the Southern Chichibu Belt in structural position, and the two units are similar in being a mixed facies and have overlapping accretionary age. However, the Otori Unit lacks limestone and has a considerably younger age for siliceous mudstone, suggesting that the two units may not be strictly correlative.