Taphonomic study of Fortipecten takahashii

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The Pliocene Horokaoshirarika Formation of the Fukagawa Group distributed in the central Hokkaido yields abundant molluscan fossils. This formation consists mainly of massive fine-grained sandstone and sandy siltstone both of which are intensely bioturbated and lose their original sedimentary structures. The formation is informally divided into upper, middle, and lower parts, on the basis of both the lithofacies and the mode of occurrences of the fossils. I divided the shell-concentrated beds of the middle part into the following five types.Type A : Mya occurs abundantly in articulated condition. Bioclasts are supported each other. Beds yield Rosselia sp, many shell fragments, and barnacles. Mya keeps their life position, and is, therefore, considered to be autochtonous. No erosional evidence is recognized in the basal part of the bed. Type B : An erosional surface is evidently observed in the basal part ofthe bed. The other characteristics are identical to the type A.Type C : Turritel … More la, Acila, and Fortipecten occurabundantly in association with Rosselia, shell fragments, and barnacles. Theyare supported by fine matrix. Fossils are considered to be more or less reworked by biological or physical agents because of their low rate of articulation and random shelll orientation in the outcrop. Type D : Fortipecten occurs ixclusively in matrix supported condition. Fortipecten shows low rate of articulation and random shell orientation, and is considered to be, therefore, more or less reworked by biological or physical agents. Type E : Species composition is nearly identical with the type C,but the fossils of this type relatively occur sporadically. The rate of articulation is low.The pale-bathymetry estimated from the molluscan and trace fossils indicates that the types A and B are deposited between subtidal and 10 m, types C and E are deeper than 30 m, and type D is deeper than 35m in depth .Seven sedimentary cycles are recognized in mud content fluctuations. The mud content is gradually higher from the base to the top in each sevencycle and is interpreted to be influenced by the sea-level fluctuation. Namely, each cycle begins shallower conditions and ends in deeper ones. The paleobathymetry estimated bu the fossils is concordant with the mud content fluctuations. Less