Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.72 No.4 (2021)

Special issue:Geology and fossils of the Kiryu and Ashikaga District, southwestern Ashio Mountains

Cover photograph | Table of Contents | Abstract

Cover photograph

Four seasons in the “Kiryu and Ashikaga” District in the southwestern Ashio Mountains

Jurassic accretionary complex is widely distributed over the Ashio Mountains across Gunma and Tochigi prefectures. The Jurassic accretionary complex in the “Kiryu and Ashikaga” District, located in the southwestern Ashio Mountains, comprises the Kurohone–Kiryu, Omama, Kuzu and Gyodosan complexes.

Upper left: (Spring) A ridge composed of chert of the Kuzu Complex in the south of Mt. Ryogai, Honjo, Ashikaga City, Tochigi Prefecture. The Kuzu Complex is characterized by a widely distributed laterally-continuous chert, which is often exposed in crests and ridges of the mountains.
Upper right: (Summer) A stream in the northern Hachioji Hills, where pelitic mixed rock of the Gyodosan Complex is distributed, Hirosawa-cho, Kiryu City, Gunma Prefecture. The Gyodosan Complex is mainly composed of chert and pelitic mixed rock, and valleys and streams mostly develop in the softer pelitic mixed rock.
Lower left: (Autumn) The Takatsudo Gorge where pelitic mixed rock of the Omama Complex is distributed, Omama-machi Takatsudo, Midori City, Gunma Prefecture. The pelitic mixed rock, which characterizes the upper part of the Omama Complex, is well exposed in the gorge. The rock and blocks of basaltic rocks, carbonate rocks, chert and sandstone in it are observed along the Takatsudo Walkway.
Lower right: (Winter) A ridge in the north of Chikasawa Pass, distribution area of mudstone of the Kurohone–Kiryu Complex, Sakuhara-cho, Sano City, Tochigi Prefecture. As the mudstone is harder than the mudstone of the other complexes, it is exposed in ridges like chert.

(Photograph and Caption by ITO Tsuyoshi)

Table of Contents

All the pages PDF ： 72_04_full.pdf ［67MB］

Title	Author	PDF
Preface
Special issue on geology of the Ashio belt of the Kiryu and Ashikaga District (Quadrangle series 1:50,000)	ITO Tsuyoshi (p191-200)	72_04_01.pdf [2.1MB]
Article
Geology and correlation of Jurassic accretionary complex in the Ashio Mountains, central Japan: Investigation on the Kiryu and Ashikaga District (Quadrangle series 1:50,000)	ITO Tsuyoshi (p201-285)	72_04_02.pdf［35MB］ Correction in Approval number, p.248 (2021.10.13) Correction of irregular color in Fig.6, p.215 (2021.10.18)
Report
Radiolarians from Jurassic accretionary complex of the Ashio belt in the Kiryu and Ashikaga District (Quadrangle series 1:50,000), central Japan	ITO Tsuyoshi (p287-324)	72_04_03.pdf ［16MB］ Correction in Approval number, p.291 (2021.10.13)
Conodont fossils from the Kiryu and Ashikaga District (Quadrangle series 1:50,000), central Japan with emphasis on the reexamination of “Carboniferous” conodonts from the Ashio Belt	MUTO Shun and ITO Tsuyoshi (p325-344)	72_04_04.pdf ［5MB］ Correction in Approval number, p.329 (2021.10.13)
Note and Comment
Occurrence report of Triassic and Jurassic radiolarians from the Jurassic accretionary complexes of the Ashio belt in eastern Mt. Narukami, Ashio Mountains, central Japan	ITO Tsuyoshi, NAKAMURA Kazuya, HINOHARA Tatsuya and KURIHARA Toshiyuki (p345-358)	72_04_05.pdf ［5.2MB］
Radiolarians and foraminifers from the Omama Complex of Jurassic accretionary complex in the Ashio Mountains, central Japan	ITO Tsuyoshi, SUZUKI Noritoshi and SASHIDA Katsuo (p359-370)	72_04_06.pdf ［5.4MB］ Correction in Approval number, p.361 (2021.10.13)
Report
Geochemical characteristics of basaltic rocks from the Omama Complex of Jurassic accretionary complex in the Ashio Mountains, central Japan	ITO Tsuyoshi and KUSANO Yuki (p371-381)	72_04_07.pdf ［4MB］ Correction in Approval number, p.373 (2021.10.13)
Note and Comment
Biotite granodiorite of the Ashikaga Body and contact metamorphic rock in Nagusa, Ashikaga City, Tochigi Prefecture, central Japan	ITO Tsuyoshi and NAKAMURA Yoshihiro (p381-396)	72_04_08.pdf ［10MB］

Abstract

Geology and correlation of Jurassic accretionary complex in the Ashio Mountains, central Japan: Investigation on the Kiryu and Ashikaga District (Quadrangle series 1:50,000)

ITO Tsuyoshi

Jurassic accretionary complex of the Ashio belt is exposed in the Ashio Mountains, central Japan. Lithology, stratigraphy, fossil age and geologic structure are summarized in this article, on the basis of the geological survey of the Kiryu and Ashikaga District (Quadrangle series 1:50,000) and previous studies in adjacent districts. The Jurassic accretionary complex in the Ashio Mountains comprises four tectonostratigraphic units, namely the Kurohone–Kiryu, Omama, Kuzu and Gyodosan (newly named) complexes. The Kurohone–Kiryu Complex presents broken to coherent facies and is composed mainly of mudstone and chert and moderate amounts of siliceous claystone, with small amounts of basaltic rocks, carbonate rocks, siliceous mudstone, sandstone and pelitic mixed rock. This complex is characterized by dominant of pelitic rock that cleavage develops. This complex is subdivided into the Upper and Lower parts. The Omama Complex presents broken to mélange facies, and consists mainly of basaltic rocks, chert and mudstone, with small amounts of carbonate rocks, siliceous mudstone, sandstone and pelitic mixed rock. This complex is subdivided into the Upper and Lower parts, and the pelitic mixed rock are dominant in the Upper part. The Kuzu Complex is subdivided into three units (Units 1, 2 and 3). Units 1 and 3 are characterized by coherent facies of chert–clastic sequence comprising chert, siliceous mudstone, mudstone, alternations of sandstone and mudstone and sandstone, in ascending order. Unit 2 comprises basaltic rocks and carbonate rocks with conglomerate, siliceous mudstone and mudstone. The Gyodosan Complex presents mélange facies and is composed mainly of pelitic mixed rock and chert, with siliceous mudstone, mudstone and sandstone. Three faults as complex boundaries were recognized: the Kiryugawa Fault (between the Kurohone–Kiryu and Omama complexes), Kanma Fault (newly named: between the Kurohone–Kiryu and Kuzu complexes) and Oiwa Fault (newly named: between the Kuzu and Gyodosan complexes). Geologic structure of the Jurassic accretionary complex is characterized by several folds (e.g. Umeda Syncline, Hikoma Anticline, Kuzu Syncline) having NE–SW axial trace. On the basis of radiolarian fossils from pelitic rock of each complex, the accretionary age of the Omama and Gyodosan is after the middle Middle Jurassic, and that of the Kurohone–Kiryu Complex and Unit 2 of the Kuzu Complex is after the late Middle Jurassic. The accretionary age of units 1 and 3 of the Kuzu Complex is after the early Late Jurassic. Based on the comparison with tectonostratigraphic units of the Mino belt, the Kurohone–Kiryu Complex can be correlated to the Nabi and Shimashima complexes; the Omama Complex is correlated to the Funafuseyama and Shirahone complexes; the Kuzu Complex is correlated to the Kamiaso and Sawando complexes. The Gyodosan Complex resembles the Kuze Complex in including Permian chert: however, the Kuze Complex includes basaltic rocks and carbonate rocks whereas the Gyodosan Complex lacks these rocks.

Radiolarians from Jurassic accretionary complex of the Ashio belt in the Kiryu and Ashikaga District (Quadrangle series 1:50,000), central Japan

ITO Tsuyoshi

Jurassic accretionary complex of the Ashio belt is distributed in the Ashio Mountains, central Japan. The Jurassic accretionary complex is composed of the Kurohone–Kiryu, Omama, Kuzu and Gyodosan complexes. This article presents new radiolarian occurrences from 42 samples of the Jurassic accretionary complex in the Kiryu and Ashikaga District (Quadrangle series 1:50,000). Permian radiolarians occurred in nine chert samples of the Gyodosan Complex. Triassic radiolarians were obtained from four chert samples of the Kuzu Complex. Jurassic radiolarians occurred in one siliceous mudstone sample of the Kurohone–Kiryu Complex; two chert, four siliceous mudstone and two mudstone samples of the Kuzu Complex; and one mudstone sample of the Gyodosan Complex. In addition, the fossil occurrences and their ages are compiled by present and previous studies on the Jurassic accretionary complex in the Ashio Mountains. On that basis, the ocean plate stratigraphy of each complex is reconstructed.

Conodont fossils from the Kiryu and Ashikaga District (Quadrangle series 1:50,000), central Japan with emphasis on the reexamination of “Carboniferous” conodonts from the Ashio Belt

MUTO Shun and ITO Tsuyoshi

Many conodonts have been reported from the Ashio Mountains, but there have been little attempts to update the information of the specimens based on the present knowledge of conodont taxonomy and biostratigraphy. This study revisits conodont specimens reported from the Kiryu and Ashikaga District in addition to presenting a few newly obtained Early Triassic conodonts. Previously published illustrations allowed reidentification of some conodonts. The geological age of some specimens was revised based on the reidentification. Notably, many of the conodonts previously considered as “Carboniferous” species were revealed to be Permian or Triassic species, and no Carboniferous species were confirmed.

Occurrence report of Triassic and Jurassic radiolarians from the Jurassic accretionary complexes of the Ashio belt in eastern Mt. Narukami, Ashio Mountains, central Japan

ITO Tsuyoshi, NAKAMURA Kazuya, HINOHARA Tatsuya and KURIHARA Toshiyuki

This article reports radiolarian occurrences from the Omama and Kurohone–Kiryu complexes of Jurassic accretionary complex of the Ashio belt, in eastern Mt. Narukami, Ashio Mountains, central Japan. Triassic radiolarians and conodont fragments were obtained from chert recovered from the Omama Complex. Bajocian and early Bathonian (Middle Jurassic) radiolarians were also extracted from mudstone in both the Omama and Kurohone–Kiryu complexes. In previous studies, Bajocian radiolarians had represented the youngest samples recovered from the mudstone at these complexes. Consequently, the lower Bathonian mudstone reported in the present study constitutes the youngest rock recovered from the Omama and Kurohone–Kiryu complexes to date.

Radiolarians and foraminifers from the Omama Complex of Jurassic accretionary complex in the Ashio Mountains, central Japan

ITO Tsuyoshi, SUZUKI Noritoshi and SASHIDA Katsuo

Radiolarian and foraminiferal occurrences from the Omama Complex of Jurassic accretionary complexes of the Ashio belt, which distributed mainly in Omama-machi, Midori City, Gunma Prefecture, are reported in this article. Guadalupian–Lopingian (middle–late Permian), Middle–Late Triassic and Jurassic radiolarians occurred in chert; Early and Middle Jurassic radiolarians occurred in siliceous mudstone. Cisuralian–Guadalupian (early–middle Permian) fusulinids and smaller foraminifers occurred in limestone.

Geochemical characteristics of basaltic rocks from the Omama Complex of Jurassic accretionary complex in the Ashio Mountains, central Japan

ITO Tsuyoshi and KUSANO Yuki

The Omama Complex, a Jurassic accretionary complex of the Ashio belt, is distributed over the western Ashio Mountains, and is characterized by dominance of basaltic rocks and chert. This study revealed the major and trace elements of these basaltic rocks. The results from geochemical analyses suggested that the basaltic rocks belonged to Enriched mid-ocean ridge basalt (E-MORB) and ocean island basalt (OIB) affinities. Such geochemical characteristics of the basaltic rocks within the Omama Complex resemble to those of the Haiya and Kumogahata complexes of the Tamba belt. Previous studies considered that the Omama Complex can be correlated to the Haiya or Kumogahata Complex based on reconstructed Ocean Plate Stratigraphy, and the geochemistry of basaltic rocks supports the geological correlation. Meanwhile, another previous study considered that the Omama Complex can be correlated to Unit 2 of the Kuzu Complex in the eastern Ashio Mountains. However, the present result does not necessarily support the consideration because the previously-reported basaltic rocks of the Kuzu Complex resemble solely OIB.

Biotite granodiorite of the Ashikaga Body and contact metamorphic rock in Nagusa, Ashikaga City, Tochigi Prefecture, central Japan

ITO Tsuyoshi and NAKAMURA Yoshihiro

Biotite granodiorite, called as the Ashikaga Body, is distributed in Nagusa, Ashikaga City, Tochigi Prefecture, central Japan. This article describes the biotite granodiorite and its surrounding rocks belonging to Jurassic accretionary complex of the Ashio belt. The biotite granodiorite shows equigranular texture. The major constituent minerals are quartz, plagioclase, K-feldspar and biotite. Metapelite and metachert has been formed by contact metamorphism in the aureole of the Ashikaga granodiorite. Chert from sites over 1.5 km from the Ashikaga Body yielded radiolarian fossils. One sample yielded Pseudoristola sp. and Archaeospongoprunum sp. which represent the Pliensbachian–early Toarcian (Early Jurassic) in age. Another sample yielded closed nassellarians, which occurs mainly in the Jurassic–Cretaceous.
The peak metamorphic temperatures of the northern and southern margin of the Ashikaga Body were estimated to be 385 ± 9 °C and 513 ± 33 °C, respectively, based on Raman spectra of carbonaceous material thermometry. The difference in metamorphic temperature between the northern and southern margin of the Ashikaga Body may be reflected by the intrusion angle of the body. A sample ca. 1 km southwest from the Ashikaga Body shows 291 ± 15 °C of peak metamorphic temperatures whereas another sample over 1.3 km northwest from the body is 362 ± 16 °C. Such high temperature suggests the existence of any concealed or already-eroded rock body.