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The younger Cenozoic strata, the Shibikawa through the Katanishi formation, on the north coast of the Oga peninsula, north Japan, were stratigraphically examined through the detailed field observation... of lithostratigraphy, particle-size analysis of the sediment samples and discrimination of the stratigraphic range and frequency of the fossil species obtained. The field observation clarified cyclic sedimentation and a few morphological unconformities in that sequence. Particle-size analysis confirmed the cyclic sedimentation in more refined sense. Furthermore it suggested that the Anden and the Katanishi formation are represented chiefly by off-shore sand and a transition from shore sand to dune sand, respectively, and that the sediments of these types are quite different from those of the Shibikawa. Almost all the molluscan species obtained from the formations in question range through Quaternary except for only three extinct ones. Accordingly any difference in fossil assemblages by horizon must be reflected by the ecological and/or sedimentary environmental conditions. Several faunal gaps in this sense were recognized. Among them the gaps between the Shibikawa and Anden and the Anden and Katanishi are much more remarkable than the others. Concerning with the evolutionary stratigraphy, the Shibikawa and the Anden formation are respectively included in Turritella otukai zone and T. andenensis zone. The zonal index T. andenesis a direct discendant of T. otukai. Under such circumstance mentioned above, the Shibikawa, Anden and Katanishi are reasonably distinguished stratigraphicaly and they should be treated as the separate formations.続きを見る
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