Toyonishi Group

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Toyonishi Group
Stratigraphic range: Tithonian to Lower Hauterivian
TypeGroup
Sub-units,
UnderliesToyora Group
Overlies
Thickness300–900 metres (980–2,950 ft)
Lithology
PrimaryMudstone, Sandstone
OtherSiltstone, conglomerate, limestone
Location
Region
CountryJapan
Type section
Named for　(Village)
Named byMatsumoto, 1949

The Toyonishi Group is a group of Mesozoic rock strata in Japan, and was originally named by in 1949.[1] It distributes in the southern half of Shimonoseki City, Yamaguchi Prefeture, southwest Japan, and deposited during the Uppermost Jurassic-Lower Cretaceous along the East Asian continental margin. [2]

Geology[]

The Toyonishi Group is approximately 300–900 m thick, lies with a disconformity or a locally angular unconformity on the Jurassic Toyora Group, and is overlain with an unconformity by the Early Cretaceous . It has been divided into the Kiyosue Formation and overlying formations, and exposes in the following three districts: Ohchi-Utsui-Kikugawa, Yoshimo, and Murotsu. The mountainous Ohchi(former Kiyosue) and west Yoshimo coastal areas are type areas of the Kiyosue Formation and the Yoshimo Formation, respectively.[2][3][4][5]

Marine coral-bearing limestone was reported by Kenichi Yoshidomi and Yasuko Inoue from the stratigraphically lower level of the Yoshimo Formation containing a brackish water fauna in the Murotsu district that is located to the north of the Yoshimo coast, and it was ascertained that this limestone-bearing unit was stratigraphically located between the Kiyosue Formation and the Yoshimo Formation.[6] However, its stratigraphic division has not yet been clarified[7]

The Toyonishi Group has been considered to Tithonian-Early Hauterivian in age. This estimate is mainly based on biostratigraphic correlations between a brackish water fauna of the Yoshimo Formation and coeval faunas in Japan.[8] The sediments of the group begin with the basal part of the Kiyosue Formation that is characterized by a thick fluvial sandstone-conglomerate bed. The main part of the Kiyosue Formation is composed mainly of deltaic mudstone, sandy mudstone, sandstone and conglomerate and the Yoshimo Formation consists of quartz sandstone and mudstone deposited in a delta and barrier-island complex.[5]

Fossils[]

The main part of the Kiyosue Formation frequently yields plant macrofossils. This plant fossil assemblage is called the ,[9] which includes both and taxa. The Yoshimo Formation yields many brackish water bivalves and gastropods that is called the , which is correlated with the in the outer zone of Southwest Japan.[3]

References[]

  1. ^ Matsumoto, T., 1949, The Late Mesozoic geological history in the Nagato Province, southwest Japan, Japan. Jour. Geol. Geogr., 21, pp. 235-243. OCLC: 610397819
  2. ^ a b Kawamura, H., 2010, Stratigraphic revision of the Jurassic Toyora Group of the southern part of the Tabe basin, Yamaguchi Prefecture, southwest Japan. Jour. Geol. Soc. Japan, 116, pp. 27-44. (in Japanese with English abstract) doi: 10.5575/geosoc.116.27 ISSN 0016-7630
  3. ^ a b Matsumoto, T., 1954, The Cretaceous System in the Japanese Islands. The Japanese Soc. Promo. Sci. Res., Tokyo, 324 p. OCLC: 191970562
  4. ^ Hase, A., 1958, Stratigraphy and Structures of the Late Mesozoic in Western Chugoku and Kitakyushu. Geol. Rep. Hiroshima Univ., 6, pp. 1-50. (in Japanese) OCLC: 883672216
  5. ^ a b Yoshidomi, K., 2003, Basin analysis of the Lower Cretaceous Toyonishi and Kanmon Groups, Southwest Japan. Jour. Sci. Hiroshima Univ. Ser. C, 11, pp. 155-188. OCLC: 656082328
  6. ^ Yoshidomi, K. and Inoue, Y., 2001, Late Mesozoic coral-bearing limestone in the Murotsu area, Toyoura Town, western Yamaguchi Prefecture, Southwest Japan. Jour. Geol. Soc. Japan, 107, pp. 794-797. (in Japanese with English abstract) doi: 10.5575/geosoc.107.794 ISSN 0016-7630
  7. ^ Yoshidomi, K., 2009, Cretaceous System-Toyonishi Group. In Geological Society of Japan ed., Regional Geology of Japan 6-Chugoku district. Asakura Publ., Tokyo, pp. 100-102. (in Japanese) ISBN 978-4-254-16786-3, OCLC 675262150
  8. ^ Matsumoto, T., Obata, I., Tashiro, M., Ota, Y., Tamura, M., Matsukawa, M. and Tanaka, H., 1982, Correlation of marine and non-marine formations in Cretaceous of Japan. Fossils, 31, pp. 26. (in Japanese) ISSN 0022-9202 OCLC: 5176840013
  9. ^ Kimura, T., 1980, The present status of the Mesozoic land floras of Japan. In Igo, H. and Noda, H. eds., Professor Saburo Kanno Memorial Volume, the University of Tsukuba, Ibaraki, pp. 379-413. OCLC: 249215238
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