化石密集層形成における堆積学的制約と古環境について : 北海道上部蝦夷層群 (上部白亜系) から産出する Inoceramus uwajimensis Yehara を例に

抄録

Biostratinomic processes, which include sedimentologic processes such as fragmentation, re-orientation, size-sorting, disarticulation and abrasion, are easily read from the evidence of fossil records. The formation of shell-concentration (Inoceramus uwajimensis bed) in the black mudstone of the Upper Yezo Group in the study area was controlled by the following three factors : (1) physical control such as resedimentation by bottom current and shell-accumulation by winnowing of fine sediments, (2) ecological factors, and (3) paleoenvironmental factor concerning the dissolved oxygen level which can be reconstructed from trace fossil assemblage. Concentrations of inoceramid shells are common and remarkable features in the monotonous Upper Cretaceous mudstone of the study area. Five types of shell beds, namely Type 1 to Type 5, are distinguished on the basis of the mode of occurrence. Most of the shell beds are discontinuous and fade away laterally. The shell beds of Types 1 and 2 consist of articulate and less abraded shells with the left-valve down, which suggests these two shell beds consist of autochthonous shells and the life positions are preserved. "Clump-like" modes of occurrence can be observed in Type 3, but the shells were not in life position as a result of subtle in situ reworking. Shell-size is totally even in such clumps. This suggests that they formed a colony which consists of individuals of the same generation as seen in recent Mytilus colonies. Type 4 usually consists of graded shell fragments and less broken shells of convex-down position at the bottom of shell beds. Type 4 undoubtedly represents an allochthonous mode of occurrence. Type 5 contains mixed fauna which consists of epifaunal and infaunal bivalves, benthic heteromorph ammonites, planktonic ammonites and planktonic foraminifera. In spite of the intense reworking, it suggests very rapid deposition after the storm that mixed faunas of different modes of life preserved very well. The assemblage of all the shell beds except for type 5 shows very low diversity : dominant I. uwajimensis plus a few other bivalves and/or gastropods. Trace fossil assemblages in the ambient black mudstone indicate an oxygen-deficient condition. One reason why I. uwajimensis shows a monotypic composition may be due to some ecological factors. I. uwajimensis seems to have adapted to such oxygen-poor environments below the intense storm-wave base where was hard for most of the other animals to live.