西南北海道黒松内地域の瀬棚層の貝類化石群

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タイトル別名
  • Molluscan fauna from the Setana Formation in the Kuromatsunai district, southwestern Hokkaido, Japan
  • セイナン ホッカイドウ クロマツナイ チイキ ノ セタナソウ ノ カイルイ カ

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抄録

The Setana Formation is widely distributed in southwestern Hokkaido and yields numerous and diversified molluscan fossils that seem to be of early Pleistocene in age. This molluscan fauna is mainly characterized by cold water elements and belongs to the so-called Omma-Manganji fauna in the Japan Sea side of northern Honshu. In the Kuromatsunai district, the Setana Formation is unconformably underlain by the Kuromatsunai Formation and overlain by the Chiraigawa Formation. From the lithologic features, this formation is subdivided into two members in the eastern part and three members in the western part of the studied district. The molluscan fauna is subdivided into three assemblages-I, II and III, which are characterized by the stratigraphic position as well as the specific composition respectively. The assemblage I, which is obtained from the lower part of this formation, consists of 20 species including the elements of the Setana fauna proper such as Yabepecten tokunagai and Chlamys islandicus, and includes about 25.0% extinct species. The assemblage II, which is obtained from the middle part of this formation, comprises 53 species and is characterized by some extinct species such as Chlamys cosibensis and C. daishakaensis. The assemblage III, which is obtained from the upper part of this formation, is characterized by large individual numbers of Mizuhopecten tokyoensis (s. s.). The assemblage III consists of 108 species which is chiefly composed of Oyashio living species such as Mizuhopecten yessoensis, Swiftopecten swiftii and Turritella fortilirata. About 5.6% extinct species are included in this assemblage. From the representative species and the ratio of extinct species, the assemblages I and II are rather in close association with each other and are corresponded to the Setana fauna proper. On the other hand, the assemblage III appears to be much younger in age than the Setana fauna proper (assemblages I and II). Namely, the abovementioned faunal change is most likely attributed to the change of environmental conditions with the lapse of the geologic time.

収録刊行物

  • 地球科学

    地球科学 43 (5), 277-289, 1989

    地学団体研究会

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