Toyonishi Group

Toyonishi Group
Stratigraphic range: Tithonian to Lower Hauterivian
TypeGroup
Sub-unitsKiyosue Formation, Yoshimo Formation
UnderliesToyora Group
OverliesKanmon Group
Thickness300–900 metres (980–2,950 ft)
Lithology
PrimaryMudstone, Sandstone
OtherSiltstone, conglomerate, limestone
Location
RegionShimonoseki City, Yamaguchi
CountryJapan
Type section
Named forToyonishi-son　(Village)
Named byMatsumoto, 1949

The Toyonishi Group is a group of Mesozoic rock strata in Japan, and was originally named by Tatsuro Matsumoto in 1949.[1] It distributes in the southern half of Shimonoseki City, Yamaguchi Prefecture, 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 Kanmon Group. It has been divided into the Kiyosue Formation and overlying Yoshimo 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 Kiyosue flora,[9] which includes both Ryoseki type and Tetori type taxa. The Yoshimo Formation yields many brackish water bivalves and gastropods that is called the Yoshimo fauna, which is correlated with the Ryoseki fauna 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 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