粒径測定による有孔虫化石殻の分析:タービダイトが卓越する斜面盆地のケース

抄録

In this study, grain-sizes of foraminiferal fossil tests were analyzed for the offshore mudstones of the Ukari and Horinouchi Formations of the Pliocene‒Pleistocene Kakegawa Group that contains the deposits of turbidite dominated fore-arc slope basin. Diameters of individual foraminiferal tests and terrigenous sand grains obtained from the sediments were calculated from their settling velocities. Comparison of the grain-size distributions of foraminiferal tests and sand grains revealed that there were two size-distribution groups of benthic foraminifers in different origin. The first group, in which sand grains and shell-tests of benthic foraminifers share the similar grain-size distributional patterns, suggests an allochthonous assemblage transported by turbidity current. It is observed in the Ukari Formation. The second group of benthic foraminiferal tests larger than the maximum size of the sand grains cannot be simply interpreted as allochthonous but could be possibly indigenous. It was found from the Horinouchi Formation. In this case, it is difficult to evaluate the origin for the group of benthic foraminiferal tests smaller than the maximum size of the sand grains. The grain-size measurements seem effective to evaluate fossil foraminiferal assemblages in combination with the other methods, e.g., modern marine ecology, geochemistry, and taphonomy.