Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.74 No.1 (2023)

Cover photograph | Table of Contents | Abstract

Cover photograph

Deep-sea Permian–Triassic boundary in the Okoshizawa Subunit (Figure is an outline sketch)

The Otori Unit of the North Kitakami Belt studied in this report includes in its structurally lower part pelagic deep-sea strata that includes the Permian–Triassic boundary. The boundary interval is composed of black claystone and siliceous claystone in which deformation of the deep-sea strata is typically concentrated. Therefore, this boundary is only known from several outcrops in Japan, despite the wide distribution of Jurassic accretionary complexes. The Otori Unit is an exceptional case, in which the Permian–Triassic boundary black claystone is traced laterally over a wide area. The outcrop along the Orikabe River in the photograph is one of the outcrops where this lithofacies is observed. As typical of this area, rock exposure is obscured by vegetation even by the streams, but a conformable relationship between Permian chert and basal Triassic black claystone is clearly seen underwater.

(Photograph and Caption by MUTO Shun)

Table of Contents

All the pages PDF ： 74_01_full.pdf ［24MB］

Title	Author	PDF
Article
Geology and accretionary age of the Otori Unit, North Kitakami Belt	MUTO Shun, ITO Tsuyoshi and MURAYAMA Masafumi (p1-40)	74_01_01.pdf [21MB]
Abstract
Abstracts of the 34th GSJ symposium “Research of Earthquake, Tsunami, and Volcano at AIST toward disaster prevention and mitigation: Achievements of 10 years since the Great East Japan Earthquake and future research”	Secretariat of the 34th GSJ Symposium (p41-60)	74_01_02.pdf［2.5MB］

Abstract

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

MUTO Shun, ITO Tsuyoshi and MURAYAMA Masafumi

 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.