Radiolarian biostratigraphy of the Middle Miocene Kinone and Amatsu Formations, Boso Peninsula, Japan, with a synthesis of siliceous and calcareous microfossil stratigraphy.

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The radiolarian biostratigraphy on the Middle Miocene marine sequence in the Boso Peninsula was firstly established through this work, which enables us to discuss the direct stratigraphic relations between siliceous and calcareous microfossil biostratigraphy. Calcareous nodules in hemipelagic mudstones of the Kinone and Amatsu Formations contain Middle Miocene radiolarian species characterizing the low- and high-latitude North Pacific regions. The low-latitude species, including Dorcadospyris alata, D. dentata, Didymocyrtis tubaria, D. mammifera, D. laticonus, Calocycletta costata and Lithopera thornburgi indicate presence of two tropical radiolarian zones, C. costata and D. alata Zones; however, the boundary between these zones is not well determined due to the very rare occurrence of D. alata and D. dentata. The high-latitude species elements include common Eucyrtidium asanoi and E. inflatum, and rare Lychnocanoma magnacornuta. These species indicate four high-latitude zones, C. costata, E. asanoi, E. inflatum and L. magnacornuta Zones. There are also common occurrences of Cyrtocapsella tetrapera, C. cornuta, C. japonica, Stichocorys delmontensis and S. wolffii.A synthesis of the biostratigraphic studies on the Kinone and Amatsu Formations presents correlation of radiolarian, diatom, calcareous nannofossil and planktonic foraminiferal zones, providing a local biochronologic framework for Middle Miocene sediments of Japan.
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