Taphonomic study of Fortipecten takahashii

分布于北海道中部的深川群上新世Horokaoshirarika组有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，两者均受到强烈的生物扰动，失去了原有的沉积构造。根据岩相和化石的赋存方式，将地层非正式地分为上、中、下三个部分。我将中部的集壳层划分为以下五种类型。A型：Mya在关节状态下大量发生。生物碎屑相互支持。床上生长着蔷薇属，许多贝壳碎片和藤壶。Mya保持他们的生活位置，因此，被认为是自主的。在床底部没有发现侵蚀的证据。B型：在床底部明显观察到侵蚀面。其他特征与a型相同。C型：Turritel，更多的la， Acila和Fortipecten，与Rosselia，贝壳碎片和藤壶相关。它们由细矩阵支撑。化石被认为或多或少是由生物或物理因素重新加工的，因为它们的低关节率和在露头中随机的壳取向。D型：仅在矩阵支撑条件下发生。Fortipecten表现出低关节率和随机壳取向，因此被认为或多或少被生物或物理试剂重新加工。E型：物种组成与C型基本相同，但该型化石相对零星出现。发音率低。软体动物及痕迹化石的浅水深测量表明，A型和B型沉积在潮下至10 m之间，C型和E型沉积在30 m以上，D型沉积在35m以上。在泥质含量波动中识别出7个沉积旋回。在每七个旋回中，泥含量由底部到顶部逐渐升高，并被解释为受海平面波动的影响。也就是说，每个循环开始于较浅的条件，结束于较深的条件。古水深测量结果与泥质含量波动一致。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