ストーム成化石密集層の軟体動物化石における生態学的属性の層位変化 : 鮮新統掛川層群大日層の下部外浜相を例に

抄録

Molluscan fossils in storm-generated shell beds are commonly transported and damaged. Here we report a case study on assessing mollusc shell transport from original habitats to lower shoreface storm deposits. Fossil assemblages recovered from eight storm-generated shell beds in the Pliocene Dainichi Formation, Kakegawa Group, Japan, were examined. We assessed temporal changes in the relative frequency of three ecological categories (mode of life of bivalves, substrate preference, and habitat depth), using available information on modern molluscan species. The studied stratigraphic section was subdivided into two distinct units based on taxonomic compositions and the ecological categories. The lower unit (D-1, D-2, D-2') is characterized by rocky-bottom dwellers such as Area arabica, Cardita leana, Chama sp., Lima vulgaris, Collisella spp., and Siliquaria cumingii. The gravelbottom dweller Arcopsis symmetrica is more common in the lower unit. The upper unit (D-3 to D-7) can be distinguished from the lower one by a decrease in deposit feeders that are better able to escape sudden burial than suspension feeders. In this unit, suspension feeders that generally inhabit quiescent environments increase in the place of the deposit feeders. The results indicate the followings: (1) the storm-generated shell beds in the study section represent indigenous fossil assemblages; (2) thin and flat bivalves (e.g., "Macoma" spp. and Saccella spp.) exclusively deposited on the laminar surface (D-2') were more or less transported, whereas gastropods accumulated in lag deposits (D-2) of hummocky cross-stratified beds were residues after a storm event and its attendant winnowing of fine sediments; (3) paleontological evidence obtained from the lower unit indicates the existence of a rocky coast during the time when the Dainichi Formation began to deposit in the study area (Dainichi, Fukuroi City); and (4) the changes in the ecological categories from the lower to the upper unit are compatible with the upward-deepening sedimentary sequence recorded in the Dainichi Formation.