Stratigraphic revision of the Jurassic Toyora Group of the southern part of the Tabe basin, Yamaguchi Prefecture, southwest Japan.

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  • Kawamura Hiroyuki
    Kyushu Institute of Technology, Institution for Education and Research Safety and Health Promotion Office

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  • 山口県，田部盆地南部のジュラ系豊浦層群の層序の再検討
  • ヤマグチケン タベ ボンチ ナンブ ノ ジュラケイ トヨラソウグン ノ ソウ ジョ ノ サイケントウ

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Abstract

The study area is located in the southern part of the Tabe Basin, Shimonoseki City, Japan. The recently revised stratigraphy of the Toyora and Toyonishi groups is reviewed based on detailed geological research, and new findings on the sedimentary environment and geological age are inferred from facies and paleobotanical data. The Lower to Middle Jurassic Toyora Group is divided into the following four formations: Higashinagano Formation, Nishinakayama Formation, Utano Formation, and Ohchi Formation (new name), in ascending stratigraphic order. The Higashinagano to Utano Formations contain marine faunas such as ammonites, inoceramids, and other molluscs, while the Ohchi Formation yields abundant plant fossils (i.e., the so-called Utano flora).<br>The Ohchi Formation has been considered non-marine or a deltaic facies, and was recently renamed as the Kiyosue Formation of the Toyonishi Group, which unconformably overlies the Utano Formation. However, the lowermost part of the Ohchi Formation has a gradational contact with the uppermost part of the Utano Formation, which is characterized by turbidite deposits laid down in a prodelta slope environment within a bay, and the Ohchi Formation contains the same Phycosiphon burrows as those found in the Utano Formation, indicating a marine sedimentary environment. In addition, muddy conglomerate at the base of the Ohchi Formation contains mud clasts deposited in an unconsolidated or semi-consolidated condition, derived from the Utano Formation. These observations suggest that the Utano and Ohchi formations represent a series of deposits laid down during a regressive phase. The flora of the Ohchi Formation has the same composition as that of Member Ut of the Utano Formation, which has been dated to Bathonian-? Callovian. Based on this correlation and other evidence, the Ohchi Formation is considered to be Bathonian-Callovian in age. Therefore, it is considered that the Ohchi Formation is unconformably overlain by the Tithonian-Hauterivian Toyonishi Group.

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