J. Geol. Soc. Japan, Vol. 130, No. 1, p.167–168, 2024
地質雑 130（1）doi: 10.5575/geosoc.2024.0009

富山県上市町の下部中新統稲村水中地すべり堆積物

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The lower Miocene Inamura subaqueous landslide deposit at Kamiichi-machi,
Toyama Prefecture, central Japan

荒戸裕之

＊1

＊2

山本由弦

＊3

＊4

＊5

＊6

山田泰広
保柳康一
金子一夫
國香正稔
＊8
＊2
＊2
藤田将人
吉本剛瑠
関山優希

＊7

白石和也

＊1

千代延俊

Hiroyuki Arato＊1, Yuzuru Yamamoto＊2, Yasuhiro Yamada＊3, Koichi Hoyanagi＊4, Kazuo Kaneko＊5, Shonen Kunika＊6,Kazuya Shiraishi＊7, Shun Chiyonobu＊1, Masato Fujita＊8, Takeru Yoshimoto＊2, Yuki Sekiyama＊2
Akita University, 1-1, Tegata Gakuen-machi, Akita 010-8502, Japan, ＊2 神戸大学理学研究科 Kobe University, ＊3 九州大学工学研究院
Kyushu University, ＊4 信州大学 Shinshu University, ＊5 立山黒部ジオパーク協会 Tateyama-Kurobe Geopark, ＊6 黒部市吉田科学館 Kurobe Yoshida Science Museum, ＊7 海洋研究開発機構海域地震火山部門 JAMSTEC, ＊8 富山市科学博物館 Toyama Science Museum
＊1

秋田大学

Corresponding author：H. Arato, hashimotoan2@gmail.com
2024 年 2 月 12 日受付．2024 年 3 月 1 日受理．
富山県上市町稲村の廃土処分地跡（Fig. 1）において灌木を伐採し崖錐堆積物を除去したところ，南北約 80 m，東西約 70 m の範囲に下部中新統折戸凝灰岩部層の大露頭が再出現した（稲村露頭：Figs. 2and 3）．同層は，前〜中期中新世の日本海拡大期に富山県から能登半島にかけて形成された北東−南西方向のリフト帯（竹内, 2021 など）南東縁に堆積した複数の単源火山砕屑岩類と関連堆積物からなる福平層の一部層である（金子, 2001）．同層にはプロデルタで堆積した凝灰質砂岩泥岩互層が挟在し，一部は堆積後の早い時期に地すべりによって変形を受けたと解釈される（以下，稲村水中地すべり堆積物）．稲村露頭では，写真に紹介するとおりすべり面や種々の変形構造が観察され（Figs. 3, 4 and 5），海底火山活動に伴う堆積盆地縁辺における水中地すべりの滑動メカニズムを議論することができる．
謝 辞 調査にご理解とご協力をくださった英修興産有限会社，立山黒部ジオパーク協会の諸氏，有限会社きんた，鉄建建設株式会社と地元の皆様に心より感謝申し上げる．なお，当該調査には日本学術振興会科学研究費（19H02397）の一部を使用した．

Fig. 1. Location of the Inamura outcrop (a) and geological map of thearea surrounding the outcrop (b: after Kaneko, 2001). In ascending
stratigraphic order, the Neogene Iozen (Iz, 医王山層), Fukuhira (Fh, 福平層), Higashibessho (Hb, 東 別 所 層), and Otogawa (Og, 音 川 層) formations dipping to the northwest overlie pre-Cenozoic rocks in this area.
The Orido tuﬀ member (Or, 折戸凝灰岩部層), a lower part of the Fukuhira formation, dips 10°–25° north- to northwestward due to post-depositional uplift of the hinterland.

Fig. 2. Aerial photograph of the Inamura outcrop, showing large cliﬀs that open toward the south. The outcrop comprises alternating beds of tuﬀaceous
sandstone and mudstone, tuﬀ and tuﬀ breccia of the Orido tuﬀ member. Some of the alternating beds are thought to have slid along the seaﬂoor relatively soon after their deposition, dipping from back to front in this image, independent of the present-day direction of the strike and dip. Red boxes andnumbers indicate the locations of the photographs shown in Figs. 3, 4 and 5.

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Fig. 3. Photograph (taken in 2003) ofthe western cliﬀs of the Inamura outcrop. An overturned fold and ramp anticline (see the close-up images in Fig.
4a, b) are visible in the upper portionof the alternating beds of tuffaceous
sandstone and mudstone. The same interval in a downstream location (upperleft) is less deformed. The underlying
alternating beds are undeformed competent sediments. The overlying tuff
layer contains huge slumped blocks of
the alternating beds (see the close-up
image in Fig. 4b). In this photograph,the slide direction is from the lower
right to upper left.

Fig. 4. Close-up photographs of theoverturned fold (a) and ramp anticline(b) described in Fig. 3, and a high-angle overturned fold (c). In (c), the upper part of the alternating beds ofsandstone and mudstone (B2) was
thinly collapsed into an overturned
fold (3). This interval is repeated in
the underlying alternating beds (B1),becoming thinner to the east of thecliﬀ shown in Fig. 2. The top of layer
B2 was eroded by tuﬀaceous layer C.
In (c), the slide direction is from backto front.
Fig. 5. Photographs of the slide surface (a) and boudin (b). The row of
red triangles in (a) indicates the slide
surface. Pale sandstone flakes are incorporated into the mudstone justabove the surface. Sandstone below
the slide surface turned upward into
an overturned fold (yellowish-greentriangles), the upper part of which was
cut away by the slide. The pale grey
sandstone bed (S) is narrow at the center of the image shown in (b), and thespace is compensated by plastic deformation of the over- and underlying
mudstones (m).
以下は，地質学雑誌データファイルとして，J-STAGE Data< https://jstage-data.jst.go.jp/geosoc> で公開されています．

Fig. A. Aerial video of the Inamura (稲村) outcrop and surrounding landscape. The
lower Miocene Orido tuﬀ member is exposed at the outcrop, which expands generally southward. Some of the alternating beds are thought to have slid along the seaﬂoor at the time, dipping from north to south, independent of the present strike anddip direction. In this video, Toyama (富山) Plain, Toyama Bay and Noto (能登) Peninsula are seen from a distance in the northwest, with Hayatsuki (早月) River in thenortheast.
文献

金子一夫, 2001, 富山県東部に分布する中新世火山岩- 火山砕屑岩の層序と造構造史．地質雑, 107, 729–748.（Kaneko, K., Stratigraphy and geotectonic history of Miocene volcanic-volcaniclastic rocks
in the eastern part of Toyama Prefecture, central Japan. J. Geol. Soc. Japan, 107, 729-748.）
竹内 章, 2021, 富山トラフおよび周辺海域のネオテクトニクス．地質雑, 127, 145–164.（Takeuchi, A.,
2021, Neotectonics in and around Toyama Trough, Japan Sea. J. Geol. Soc. Japan, 127, 145-164.）

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