Taphonomy of the bivalve assemblages in the upper part of the Paleogene Ashiya Group, southwestern Japan
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概要
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The Paleogene Ashiya Group, in which molluscan fossils are abundant (=Ashiya fauna), consists mainly of shallow marine deposits that exhibit sedimentary cycles especially in the Waita Formation (upper part of the Group). Each cycle is redefined as a thin transgressive basal sandstone (transgressive systems tract) overlain by a progradational coarsening-upward interval (highstand systems tract). The depositional environment varies from a shallower condition influenced by strong wave action (shoreface?) to a deeper condition below the storm wave base, which is followed by next shallower conditions such as lower shoreface or intertidal zone. Molluscan fossils occur only from the thin lower part of each cycle, namely the transgressive basal sandstone and from the mudstone of the earliest progradational phase. The fossils occur both as shell concentrations and more dispersed fossiliferous deposits. Bed-by-bed sampling based on taphonomic, sedimentologic and paleoecologic observations distinguishes four fossil assemblages, (a) Glycymeris-Phacosoma, (b) Venericardia-Crassatella, (c) Venericardia and (d) Yoldia-Nucula. These assemblages occur successively in each cycle, and their taphonomic features also change upward from a wave-generated allochthonous shellbed on the basal ravinement surface to autochthonous shell patches. The successive change accompanies a decreasing wave-influence during a transgressive period. Epibionts, such as epifaunal byssally attached bivalves and barnacles, occur abundantly as associated species of the Venericardia-Crassatella assemblage from the middle part of the transgressive basal sandstone. Epibiontic colonization probably reflects taphonomic feedback, with shelly substrates avoiding burial by the winnowing of sediments during transgression. Autochthonous shellbeds dominated by Venericardia subnipponica are intercalated in the glauconitic sandstone beds (surface of maximum transgression) at the top of the transgressive basal sandstone. The shellbeds probably represent an attritional accumulation with dead shells of Venericardia supplied continuously in situ during a phase of low sediment supply.
- 2002-04-28