Taphonomic study of Fortipecten takahashii

分布于北海道中部的福川群上新统Horokaoshiarika组出土了丰富的软体动物化石。该组主要由块状细粒砂岩和砂质粉砂岩组成，两者都受到强烈的生物扰动，失去了原有的沉积结构。根据化石的岩相和产状模式，该组非正式地分为上、中、下三部分。笔者将中部贝壳聚集层划分为以下五种类型：A型：Mya在节理条件下大量赋存。生物碎屑相互支撑。床层产生Rosselia sp，许多贝壳碎片和藤壶。MYA保持了它们的生命地位，因此被认为是自体的。在床的底部没有发现侵蚀迹象。B型：床底部有明显的侵蚀面。其他特征与A型相同。C型：Turritel…更多的La、Acila和Fortiecten与Rosselia、贝壳碎片和藤壶有关。它们由精细的矩阵支撑。化石被认为或多或少地被生物或物理因素重新加工，因为它们在露头中的结合率低，贝壳取向随机。D型：在基质支持的条件下，强韧蛋白单独出现。坚果蛋白的结合率低，壳层取向随机，因此被认为或多或少是由生物或物理因素重新加工的。E型：物种组成与C型几乎相同，但这种类型的化石相对零星地出现。由软体动物化石和遗迹化石估算的浅水深表明，A型和B型沉积在潮下至10m之间，C型和E型沉积在30m以上，D型沉积在35m以上，泥质含量波动可识别出7个沉积旋回。各旋回泥质含量自下而上逐渐升高，解释为受海平面波动影响。也就是说，每个周期开始于较浅的条件，结束于较深的条件。根据化石估算的古生物测量值与泥质含量的波动相吻合。较少

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