Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.72 No.2 (2021)

Special issue: New data from the North Kitakami Belt in the Kitakami Massif, Iwate Prefecture: On the Sotoyama District, Quadrangle Series 1:50,000

Cover photograph | Table of Contents | Abstract

Cover photograph

Mts. Iwate and Himekami seen from the top of Mt. Iwagami, Iwate Prefecture

Mt. Iwagami (alt. 1,103 m), Morioka City in Iwate Prefecture, consists of the Early Jurassic accretionary complex in the North Kitakami Belt. Greenish gray chert, contact-metamorphosed by the underlying Early Cretaceous granite, crops out on the top of the mountain as is often the case with peaks and ridges in this area. The active volcano Mt. Iwate (alt. 2,038 m) is seen in the west and Mt. Himekami (alt. 1,124 m) of Early Cretaceous granite in the northwest. The Kitakami River flows from north to south in the Kitakami lowland between the two mountains. Mts. Iwate, Himekami and Hayachine are regarded as the three major mountains in Iwate Prefecture. According to a folktale, Mt. Himekami is the wife of Mt. Iwate and Mt. Hayachine is his mistress.

(Figure and Caption by UCHINO Takayuki)

Table of Contents

All the pages PDF ： 72_02_full.pdf ［46MB］

Title	Author	PDF
Preface
Significance of the Sotoyama District, quadrangle series 1:50,000, and the special issue on its primary data	UCHINO Takayuki (p95-97)	72_02_01.pdf [1.5MB]
Report
Middle Jurassic zircon age from sandstone within the accretionary complex in the North Kitakami Belt, Kamatsuda area in Iwaizumi Town, Iwate Prefecture, Northeast Japan: Verifying the age of the accretionary complex containing the Okawa Sample	UCHINO Takayuki (p99-107)	72_02_02.pdf［14.5MB］ p.101 l.11 Correction in name spelling (May 19, 2021) List of errata here.（September 14, 2022） errata_72_02_02.pdf ［213KB］
Note and Comment
Geochemical features and origin of basalt within the Jurassic accretionary complex in the southwestern margin of the North Kitakami Belt, Northeast Japan	UCHINO Takayuki (p109-118)	72_02_03.pdf［11MB］
Report
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 and SUZUKI Noritoshi (p119-127)	72_02_04.pdf ［6.7MB］
The Towada-Ofudo Tephra found along the Oishi River, Yabukawa, Morioka City, Iwate Prefecture	KUDO Takashi and UCHINO Takayuki (p129-138)	72_02_05.pdf ［12.6MB］ List of errata here.（November 18, 2021） errata_72_02_05.pdf ［489KB］

Abstract

Middle Jurassic zircon age from sandstone within the accretionary complex in the North Kitakami Belt, Kamatsuda area in Iwaizumi Town, Iwate Prefecture, Northeast Japan: Verifying the age of the accretionary complex containing the Okawa Sample

UCHINO Takayuki

 Uchino (2019) conducted U–Pb dating of detrital zircon from some sandstone samples in the Jurassic accretionary complex, whose terrigenous clastic rocks are unfossiliferous, in the southwestern margin of the North Kitakami Belt. As a result, the Early and Middle Jurassic accretionary complexes were recognizable. However, the youngest cluster age of ca. 260 Ma was also recognized from the sandstone ("Okawa Sample" in the Kamatsuda area, Iwaizumi Town, Iwate Prefecture) located between outcrops of the respective sandstones corresponding to the Early and Middle Jurassic. Regarding interpretation of its deposition age, the Late Permian, Late Triassic or Early Jurassic had been proposed.
 In this study, U–Pb dating of detrital zircon from sandstone near the outcrop of the Okawa Sample was conducted anew to verify the age of the Okawa Sample. As a result, a youngest cluster age of ca. 160 Ma was newly found. Thus, the accretionary complex containing the Okawa Sample in the Kamatsuda area proved to be the Middle Jurassic, not the three assumed epochs listed above.

Geochemical features and origin of basalt within the Jurassic accretionary complex in the southwestern margin of the North Kitakami Belt, Northeast Japan

UCHINO Takayuki

 Basalt within an accretionary complex is generally considered to be part of fragments scraped from a subducting oceanic plate. Regarding the basalt within the North Kitakami Belt in Northeast Japan, although it is also thought to be of oceanic islands, there have been few geochemical studies of the basalt. In this report, whole-rock geochemical analyses were preliminarily conducted on six basalt samples from the Early–Middle Jurassic accretionary complexes in the southwestern margin of the North Kitakami Belt to determine their origins. As a result, four samples showed a geochemical signature consistent with MORB, whereas two samples were alkaline oceanic island basalt, according to geochemical discrimination diagrams and trace-element spider-diagrams. Notably, a certain amount of basalt indicating the MORB signature is recognized within the North Kitakami Belt.

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 and SUZUKI Noritoshi

 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.

The Towada-Ofudo Tephra found along the Oishi River, Yabukawa, Morioka City, Iwate Prefecture

KUDO Takashi and UCHINO Takayuki

 Four tephra layers, Os-4, Os-3, Os-2 and Os-1 in ascending stratigraphic order, were found in the Quaternary valley floor deposits distributed along the Oishi River, Yabukawa, Morioka City, Iwate Prefecture. Correlation of tephra layers was carried out on the basis of stratigraphy, sedimentary facies, constituent materials and major element chemical compositions of volcanic glasses. The Os-4 is estimated to be derived from the Iwate or Akita-Komagatake volcanoes, but a detailed correlation is difficult at present. The Os-3 is correlated to the Towada-Ofudo Tephra (36 ka) and corresponds to the co-ignimbrite ash fall deposit of the Towada-Ofudo Pyroclastic Flow Deposits. The Os-2 is a reworked deposit of the Towada-Ofudo Tephra. The Os-1 is probably correlated to the Towada-Hachinohe Tephra.