Fossils

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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.

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Benthic foraminifers of the Maido Formation (Pliocene) are studied at four sections in the Ajigasawa district, Aomori Prefecture. The middle and upper parts of the formation are assigned to the Globorotalia ikebei/Orbulina universa Zone and the Globigerina pachyderma (dextral)/Globorotalia orientalis Zone, respectively. The boundary in between is recognized by the first occurrence of G. orientalis in the upper part of this formation. The benthic foraminiferal fauna from the Maido Formation is divided into two assemblages on the basis of species compositions. The Euuvigerina assemblage is dominated by Euuvigerina akitaensis (Asano). The Islandiella assemblage is characterized by Islandiella japonica (Asano and Nakamura) and Islandiella norcrossi (Cushman). The Islandiella assemblage dominates the upper part of this formation. The Euuvigerina assemblage is comparable to the Recent upper bathyal fauna around Japan. The Islandiella assemblage is regarded as a bank fauna. Assemblages dominated by the family Cassidulinidae including the Genus Islandiella are commonly found from the Pliocene and Lower Pleistocene in the Oshima Peninsula, southwest Hokkaido and Aomori Prefecture. Among them, the assemblage from Hokkaido and the Pacific coast of Aomori Prefecture is dominated by Islandiella sublimbata (Asano and Nakamura). In contrast, that on the Sea of Japan side in Aomori Prefecture is dominated by I. japonica and I. norcrossi. Such a difference may represent paleoenvironmental differences with respect to water depth or substratum.

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In the Cretaceous period there were a chain of inland basins extending from North Kyushu to the Kinki region where estherids and other lacustrine fossils occur.

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