Taphonomy of fish fossils from the Miocene Tottori Group, Southwest Japan : Part 1: Stratigraphy and geologic structure

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  • 鳥取層群産魚類化石のタフォノミー : その1:層序と地質構造
  • トットリソウグンサンギョ ルイカセキ ノ タフォノミー(ソノ 1)ソウ ジョ ト チシツ コウゾウ

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Mid-Miocene transgressive sediments at Miyanoshita, Tottori City, yield abundant fossils of shallow marine fishes. The fossil assemblage includes many species so that it has the outstanding importance to clarify the Cenozoic evolutional history of shallow marine fishes in Japan. The various preservation grades of the fish fossils and the sedimentary facies of the transgressive deposits are expected to provide a train of clues to understand the taphonomic processes of the fishes. This paper, as the first part of our taphonomic study from sedimentological aspect, reveals the stratigraphy and geologic structure of the deposits in vicinity of Miyanoshita. The major conclusions are as follows: 1) Stratigraphic units in the vicinity are of pre-Neogene basements, Lower to Middle Miocene, Pliocene, and Quaternary, divided with unconformities. The Lower to Middle Miocene Tottori Group is subdivided, in ascending order, into the Koge conglomerate, the Kawabara volcanics, the Entsuji conglomerate, sandstone and mudstone, the Shozan sandstone and mudstone, the Fuganji mudstone and sandstone, and the Tochimoto shale member. 2) The deformation of the Tottori Group is gentle in general but moderate in part. Major structure is the NNE-trending synclinal structure that is complexly modified by faults and monoclines of NW-SE and NE-SW trends. 3) Argillaceous facies of the Tottori Group is divided into the Fuganji mudstone and sandstone member (continental) and the Tochimoto shale member (marine) with a conformable contact. The fish fossil bed at Miyanoshita occurs in a landslide block, but is identifiable to be at the basal horizon of the Tochimoto shale member. Bio-stratigraphic and fission-track datings indicate that the depositional age of the fish fossil bed is the end Early Miocene (16.5-16 Ma).

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