By
Fumio YOSHIDA
*, Yoshiharu NISHIOKA
**, Katsumi KIMURA
**and Hideaki NAGAMORI
**(Written in 2002)
(ABSTRACT)
* MEXT (Ministry of Education, Culture, Sports, Science and Technology)
**Institute of Geoscience
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Fig. 1 Geological summary of the Omi-hachiman district̲̲
Kinugasa-yama, Kamewari-yama, Kakuyoku-zan and neighboring mountains. These rocks consist largely of rhyolite to dacite welded tuff and can be divided into the Azuchi, Koshigoe, Kamewariyama and Okushimayama Welded Tuffs in ascending order.
The Azuchi Tuff crops out mostly at the lower slope of the Kinugasa-yama and consists of pyroxene-hornblende rhyolite and dacite welded tuff. The Koshigoe Tuff, less than 20 meters thick, is sporadically exposed at the slope of the Kinugasa-yama and is composed of biotite rhyolite welded tuff containing a small amount of sandstone and conglomerate.
The Kamewariyama Tuff, occupying a large part of the Koto Rhyolites, is made up of biotite rhyolite welded tuff. The
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Okushimayama Tuff outcrops at the Chomeiji-yama and Kakuyoku-zan Mountains and consists of biotite-hornblende rhyolite welded tuff.
Granite porphyry and other rocks
The granite porphyry and other rocks, most probably related to the Koto Volcanism, consist of biotite granite porphyry, porphyritic biotite granite and granodiorite and are exposed in the Chomeiji-yama, Oka-yama and Kamewari-yama_
Mountains. The porphyry rock in the Chomeiji-yama has intruded into the Kamewariyama Tuff.
PLIO-PLEISTOCENE KOBIWAKO GROUP
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The Kobiwako Group is a fluviolacustrine basin-fill of the Kobiwako Sedimentary Basin that formed in the Omi and Iga-ueno basins since the Pliocene. The group, more than 1,500 meters thick, consists of mud, sand, pebbly sand, sandy gravel and gravel with many thin intercalations of volcanic ash, and yields fossil plants, a nonmarine fauna-such as Stegodon and footprints of mammals and birds.
The sediments in the district, up to 700m thick, correspond to the middle part of the whole succession of the group, and are divided into the Koka, Gamo, and Kusatsu Formations in ascending order. The three formations can be locally subdivided into several members, and correlated with each other on the basis of the tephrostratigraphic and lithostratigra-phic relationship as shown in Fig. 1.
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The Koka Formation, which outcrops in the Konan Kogyo Danchi and the Yasu-gawa, consists of abundant pebble - to cobble-sized gravel and rare sand and mud. The Gamo Formation is exposed in the Yokaichi, Hino, Minakuchi and Ritto hills and the Konan Danchi. This formation is made up of sand, mud and gravel. The Kusatsu Formation, upper formation of the Kobiwako Group in the district, crops out in the above-mentioned hills and is composed of abundant pebble-sized gravel and rare sand and mud. All the members in the Konan Danchi and along the eastern foot of the Kagami-yama Mountain are made up of the sediments derived from the adjacent Yasu Granite and Koto Ryolites. On the contrary, the deposits of the other members were most probably supplied from the Mino-Tamba Belt and Koto Rhyolites.
Two NW-SE trending major faults, named the Shimoda and Shofukuji Faults, are recognized along the foot of the Kagami-yama and Iwane-yama Mountains. These faults cut or deform the Kobiwako Group.
LATE QUATERNARY DEPOSITS
The late Quaternary deposits in the mapped district comprises 1) latest Middle to Late Pleistocene fluvial terrace deposits in the diluvial uplands and 2) alluvium (Latest Pleistocene to Holocene sediments) under the fluvial lowland.
The terrace deposits, less than 10 meters thick, are divided into higher, middle I, middle II, lower and lowermost terrace deposits from older (upper) to younger (lower). The deposits consist of abundant pebble- to cobble-sized gravel and rare
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sand. The higher terrace deposits rest on the Kobiwako Group in the Yokaichi and Minakuchi Hills, and the middle and
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lower terrace deposits lie around the Yokaichi, Hino, Minakuchi, and Ritto Hills.
The alluvium under the fluvial lowland, less than 20 meters thick, can be divided into 1) piedmont gravelly deposits, 2) lake plain, flood plain, delta plain, valley bottom plain and gentle fan deposits, 3) backmarsh deposits, 4) natural levee deposits, 5) channel bar deposits and 6) sand beach and beach ridge deposits. All the deposits consist mostly of Holocene veneer of sand, gravel and mud.
ECONOMIC AND ENVIRONMENT GEOLOGY
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Manganese deposits in the Tamba Complex were once mined at the Ioi Mine until the early 1960's in the Anyoji-yama
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in Ritto. Small deposits of copper, iron and limestone in the complex in Ishibe were once mined. Clay of the Kobiwako Group, especially clay of the Yamanaka, Hie and Shinohara Members, was dug as a raw material for pottery types named Hatta-Yaki and Shimoda-Yaki and others. Mud of the alluvium around the Kakuyoku-zan was also excavated for a rooftile called Hachiman-Kawara until recently. Chert, siliceous mudstone and mudstone in the complex are now quarried at several quarries in Ritto and Ishibe. Stone of the Yasu Granite and granite porphyry was once quarried at several
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stoneworks. A hot spring called the Junibo Onsen (or Yurara Onsen) is located at the Iwane-yama in Kosei. The borehole of this spring was deeply drilled into the Yasu Granite, and the spring welled up in 1994.
執筆分担
第1章 地 形 吉田史郎
第2章 地質概説 吉田史郎・西岡芳晴・木村克巳・長森英明
第3章 丹 波 帯 木村克己
第4章 野洲花崗岩 西岡芳晴
第5章 湖東流紋岩類及び花崗斑岩類 西岡芳時
第6章 古琵琶湖層群 吉田史郎・長森英明
第7章 第 四 系 吉田史郎
第8章 応用地質 吉田史郎
文献引用例
全体引用の場合：
吉田史郎・西岡芳晴・木村克己・長森英明（2003）近江八幡地域の地質．地域地質研究報告（5万分の1地質図幅）． 産総研地質調査総合センター，72p．
章単位での引用の場合：
吉田史郎（2003）近江八幡地域の地質，第1章 地 形，地域地質研究報告（5万分の1地質図幅）．産総研地質調査 総合センター，p l‐8．
Bibliographic reference
In the case of the whole citation :
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Yoshida, F., Nishioka, Y., Kimura, K. and Nagamori, H. (2003) Geology of the Omi-hachiman district. Quadran-̲̲
gle Series, 1 : 50,000, Geological Survey of Japan, AIST, 72p. (in Japanese with English abstract 3p.).
In the case of a partial citation :
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Yoshida, F. (2003) Geology of the Omi-hachiman district, Chapter 1, Topography, Quadrangle Series, 1 : 50,000,̲̲
Geological Survey of Japan, AIST, p.1-8. (in Japanese)