Age and significance of Miocene diatoms and diatomaceous sediments from northeast Japan

Neogene diatomaceous floras collected from marine sediments exposed in northeastern Japan provide evidence of diatom evolution, stratigraphic correlation and the geologic and paleoceanographic history of this region. Middle to late Miocene diatom zones (younger than the early middle Miocene Denticulopsis lauta Zone) have been established in the Oga Peninsula section, an onshore sequence regarded as the Neogene reference section for northeast Japan. Correlative middle and late Miocene diatom floras are not preserved in sediments recovered by Ocean Drilling Program Legs 127 and 128 in the Japan Sea, emphasizing the importance of the time-equivalent onshore deposits. Initial marine sedimentation is represented by lower middle Miocene (16–15 Ma) littoral and shelf deposits exposed along the southern and eastern margins of the Japan Sea. Subsequent middle and late Miocene deposition was marked by widespread deposition of bathyal diatomaceous sediments as recorded in the Onnagawa and correlative formations in onshore sequences. The major global cooling event at 13 Ma is signaled in the Japan Sea area by the disappearance of subtropical warm-water diatoms and the appearance of subarctic cold-water floras, an event coincident with a period of rapid subsidence in the Japan Sea. The zonal marker species and the species mentioned in the text are illustrated.