Shipboard and shore-based investigation on siliceous and calcareous microfossil biostratigraphy, magneto-stratigraphy and tephrostratigraphy identified numerous datum events from the sedimentary sequences of Sites 1150 and 1151 drilled on the forearc basin of northern Japan by the Ocean Drilling Program Leg 186. Some 83 datum events were selected to construct new age-depth models for the sites. Based on the reliable magneto-stratigraphy from the Pleistocene to the Upper Miocene, which were correlated to the standard geomagnetic polarity timescale, and on excellent records of diatom and radiolarian biostratigraphy throughout the sequences, the shipboard age model was revised. Major revisions referred to stratigraphic position of the Miocene-Pliocene boundary that has been shifted more than 200 m downward in each sequence. The age-depth relations of the forearc sites represent drastic changes in the sedimentation rate-extremely high (40 cm/k.y. on average) in the Early Pliocene and low (less than 2 cm/k.y. on average) in the Middle Miocene-and several hiatuses exist throughout the sequence. The drastic changes can be related mostly to changes in diatom sedimentation and the tectonics of the Japanese Island Arc. Local ages for some foraminiferal, calcareous nannofossil and radiolarian bioevents are estimated from the age-depth models at each site. These newly calibrated bioevents and biozones as well as established diatom biostratigraphy are incorporated into the updated magneto-biochronologic timescale, which will contribute to an improvement in biochronologic accuracy of Neogene sediments in northern Japan and adjacent areas.
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