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.

Virtual Paleobiology – Advances in X-Ray Computed Microtomography and 3D Visualization of Fossils

“…The geology of the area around Okoshizawa is detailed in Ehiro et al (2008) and Muto et al (2023) (Figure 2). The pelagic deep-sea sedimentary rocks in Okoshizawa mostly comprise bedded chert and siliceous claystone of the Otori Unit of Takahashi et al (2016) (revised by Muto et al, 2023).…”

“…The geology of the area around Okoshizawa is detailed in Ehiro et al (2008) and Muto et al (2023) (Figure 2). The pelagic deep-sea sedimentary rocks in Okoshizawa mostly comprise bedded chert and siliceous claystone of the Otori Unit of Takahashi et al (2016) (revised by Muto et al, 2023). The Otori Unit is mainly composed of stacked sheets of coherent sequences of pelagic deep-sea chert and hemipelagic siliceous mudstone in the structurally lower part (Okoshizawa Subunit), and mixed rock facies in the structurally upper part (Osakamoto Subunit) (Figure 2; Muto et al, 2023).…”

Elevated sedimentation of clastic matter in pelagic Panthalassa during the early Olenekian

“…The Akkamori-2 section (Am-2 section) is one of the deep-sea sedimentary sections covering the Permian-Triassic boundary interval in the Akkamori area, Iwate Prefecture, Northeast Japan (Takahashi et al, 2009). It belongs to the Otori Unit of Takahashi et al (2016) and Muto et al (2023) in the northeastern part of the Jurassic accretionary complex of the North Kitakami Belt (Ehiro et al, 2008; Figure 2). The Otori Unit consists of upper Carboniferous to Triassic chert (the Otori Formation of Sugimoto, 1974) and Middle Jurassic mudstone (the Osakamoto Formation of Sugimoto, 1974) and is characterized by a coherent chert-siliceous mudstone sequence in the structurally lower part and a mixed facies in the structurally upper part (Muto et al, 2023; age data given in Ehiro et al, 2008;Muto et al, 2021;Suzuki et al, 2007;Takahashi et al, 2009Takahashi et al, , 2016.…”

“…Geological map of the North Kitakami Belt in the Akka area (adopted from Muto et al, 2023 based on Ehiro et al, 2008, Nakae et al, 2021, Sugimoto, 1974). Legend for lithofacies and their tectonostratigraphic division is given in the lower left diagram…”

“…(c) Compiled lithostratigraphy of the deep-sea chert gap (adopted from Muto, 2021). Tb: Triassospathodus brevissimus Zone; Th: Triassospathodus homeri Zone; Cg: Chiosella gondolelloides Zone; Ct: Chiosella timorensis Zone; Pb: Paragondolella bulgarica Zone F I G U R E 2 Geological map of the North Kitakami Belt in the Akka area (adopted from Muto et al, 2023based on Ehiro et al, 2008, Nakae et al, 2021, Sugimoto, 1974. Legend for lithofacies and their tectonostratigraphic division is given in the lower left diagram microfossil-based age (Takahashi et al, 2010).…”