Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.76 No.1/2 (2025)

Cover photograph | Table of Contents | Abstract

Cover photograph

Four seasons in the Kado District

Upper left: (Spring) Fresh green leaves on the southern slope of Mt. Anamegadake. In the Kitakami Mountains, there can be snowfall as late as May, and a rare combination of young green leaves and fresh snow can be seen.
Upper right: (Summer) Upper reaches of the Orikabe River. In summer, leaves on the trees cut off a large part of sunlight to the stream. The forest floor is also covered by vegetation, making it difficult to observe outcrops.
Lower left: (Autumn) Autumn foliage in the mountains southwest of Sakamoto Settlement in the upper reaches of the Akka River. Beech forests that are widespread in the Kitakami Mountains produce a magnificent blanket of yellow in the autumn.
Lower right: (Winter) Nekosokomatazawa River, southwest of Hayasaka Pass. The Kado District is situated near the Pacific coast, where snow precipitation is relatively scarce in the first half of winter. However, in high altitudes like the Hayasaka Highlands, snow covers the ground from around November.

(Photograph and Caption by MUTO Shun)

Table of Contents

All the pages PDF ： 76_01_full.pdf ［65MB］

Abstract

Carboniferous and Permian conodont fossils from bedded chert in Otori, Iwaizumi Town, Iwate Prefecture, with a review of previously reported conodonts from the North Kitakami Belt

MUTO Shun, TAKAHASHI Satoshi and MURAYAMA Masafumi

Conodont biostratigraphy of pelagic deep-sea sequences in the Jurassic accretionary complex of Japan offers a globally correlative timescale for these rare sedimentary records. The northern Kitakami Mountains provide potential for conodont biostratigraphic research of the deep-sea sedimentary rocks, especially for the Paleozoic interval where zonal schemes remain incomplete. Herein, we report conodont fossils from a deep-sea section named the Otori section in Iwaizumi Town, Iwate Prefecture. Conodonts were visualized using microfocus X-ray computed tomography. We identified Mesogondolella clarki, Mesogondolella aff. donbassica, Mesogondolella cf. bisselli, Mesogondolella cf. idahoensis, Jinogondolella cf. palmata, Jinogondolella postserrata, Sweetognathus iranicus, Jinogondolella altudaensis and Jinogondolella xuanhanensis. These conodonts indicate the Moscovian (middle Pennsylvanian, Carboniferous) to the Capitanian (upper Guadalupian, Permian). We also compiled and reviewed previous reports of conodont occurrences in the northern Kitakami Mountains. While previous reports have recognized late Carboniferous to Triassic ages based on conodonts, a majority of the Permian ages are not attestable due to the lack of taxonomic descriptions and illustrations.

Middle Jurassic radiolarians from manganese nodules obtained in the western part of the Kado District, northern Kitakami Mountains

MUTO Shun, ITO Tsuyoshi and OZEKI Masanori

The accretionary history of the Jurassic accretionary complex of the North Kitakami Belt in Northeast Japan is obscured by the metamorphism of Cretaceous plutons that hinder extraction of radiolarian fossils. Some of the most successful cases of radiolarian extraction in this area treated manganese nodules. In this study, we obtained well-preserved radiolarians from three manganese nodules embedded in argillaceous rocks in the western part of the 1: 50,000 quadrangle series Kado District. The age of the radiolarian fossil assemblages is assigned to the early Bajocian and Aalenian to Bajocian (Middle Jurassic) for two samples from grey bedded mudstone and Bajocian (Middle Jurassic) for a sample from mudstone that possibly experienced tectonic mixing. The age of the assemblages approximates the accretionary age of the studied rocks. This is the first report of age diagnostic radiolarians from the western part of the Kado District.

Accretionary age of the Jurassic accretionary complex of the North Kitakami Belt:new data from zircon geochronology in the Kado District

MUTO Shun

The study on the accretionary history of the Jurassic accretionary complex of the North Kitakami Belt in Northeast Japan has been hampered by the scarcity of reports on radiolarians due to metamorphism of Cretaceous plutons. Recently, zircon geochronology is being employed to elucidate the age of accretion from strata that yield rare or no microfossils. This study reports zircon U–Pb ages from igneous zircons in tuff and detrital zircons in sandstone from the 1: 50,000 Kado District in northeast Iwate Prefecture. Based on compilation of available data, the accretionary complex distributed in the main part of the northern Kitakami Mountains is classified mainly into the following seven tectonostratigraphic units: The Kadoma Unit of Rhaetian to Middle or early Late Jurassic age, the Misugo Unit of undetermined age, the Aalenian to Bathonian Otori Unit, the Bathonian to Kimmeridgian Seki Unit the Oxfordian to Kimmeridgian Takayashiki Unit, the Kayamori Unit of undetermined age and the Kimmeridgian Ekari Unit. The former six units are structurally stacked up in this order. The exact structural position of the Ekari Unit is not clear, but it is correlated to the Takayashiki Unit or a lower and younger unit, in terms of age. The new data in this study provided constraints on the accretionary age of the Ekari, Takayashiki, Seki, Otori and Kadoma units, although detrital zircons from sandstone were not always useful. The Ekari Unit lies in fault contact between older units, meaning that the faults have vertical displacements. Such faults and kilometre-scale folds interrupt the general oceanward younging trend of the accretionary complex of the North Kitakami Belt. Zircons in tuffs have not widely been used to estimate accretionary ages, but this study shows that they can be powerful tools, especially when microfossils are difficult to obtain.

Whole-rock geochemical compositions of the Ryoke plutonic rocks and metamorphic rocks from the 1:50,000, OGAWARA Quadrangle

YAMASAKI Toru

In order to provide a geochemical basis for the lithological classifications of the plutonic rocks identified during the preparation of the 1:50,000 scale geological map of the Ogawara Quadrangle, an analysis of the major and trace elements of the rocks from the Ryoke Plutono–Metamorphic Complex within this map area was conducted. Previous studies have suggested that the plutonic rocks in the Ogawara area are similar in lithology and that in some cases it is difficult to distinguish between rock types or the existence of intermediate lithologies. The results of this study show that lithological classification based on field occurrences and petrography can also be distinguished by a combination of different indices of whole-rock chemical composition. While the whole-rock chemical compositions of these lithological types correspond to the modal compositions of the constituent minerals, they cannot be explained by a simple parent-child relationship, suggesting the existence of individual parent magmas. Furthermore, based on preliminary examinations, the compositions of these rocks are similar to those of plutonic rocks in the Akechi area of the Mikawa–Tono district, about 60 km away. This suggests that an inter-regional comparison of the lithological types can be made by combining whole-rock chemical compositions with field occurrences and petrography.