大分県津久見市無垢島の上部中生界と二枚貝化石群集

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  • オオイタケン ツクミシ ムクジマ ノ ジョウブ チュウセイカイ ト ニマイガイ カセキ グンシュウ
  • Upper Mesozoic Formations and their Bivalve Faunas in Mukujima Island, Tsukumi City, Oita Prefecture

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Mukujima Island is 16 kilometers northeast of Tsukumi City, Oita Prefecture, and is geotectonically occupied by the Chichibu Terrain of the Outer Zone of Southwest Japan. In this area the Upper Mesozoic strata are exposed in Jimukujima and Okimukujima Islands, and are lithostratigraphically divided into four formations, i.e. Mukujima, Jimukujima, Okimukujima and Bungo Formations in ascending order. In this paper, the stratigraphy is described in some detail, with remarks on correlation, and the features of the bivalve faunas are made clear. The Mukujima Formation, about 65m thick, is characterized by the predominance of feldspathic quartzsandstone. Several shallow marine bivalves which were conspecific with the bivalves from the Torinosu Group in Sakawa, Sakamoto, Yatsushiro and Tanoura areas, and Soma Group (Nakanosawa Formation) in Soma area, and the fragmental plant fossils occur from this formation. From the faunal aspects and lithological characters, the Mukujima Formation is best comparable to the Torinosu Group in Shikoku, and is assigned to the Late Jurassic. The Jimukujima Formation about 550m thick is in fault contact with the Mukujima Formation. The formation is characterized by the frequent occurrence of red-colored rocks, with intercalation of brackish-water shell beds. From the fossil-contents (Ryoseki fauna) and lithological characters, the formation is comparable to the Ryoseki Formation of the Monobegawa area, the Koshigoe Formation of the Haidateyama area, the Togawa Formation of the Gokase area and the Kohara Formation of the Yatsushiro area.

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