Radiolarian fossils from the Miocene Tsurushi Formation distributed in Sado Island, Niigata Prefecture, Japan

Kawatani Ayako, Sashida Katsuo, Agematsu Sachiko, Kohno Naoki

doi:10.9795/bullgsj.70.91

<p>The Middle Miocene Tsurushi Formation distributed in Sado Island is composed of the lower basalt and upper mudstone. This formation is important to elucidate the geohistory of Sado Island and the expansion process of Japan Sea.</p><p>We tried to clarify the geologic age of the Tsurushi Formation by using biostratigraphical data on microfossils from this formation. We identified radiolarian fossils from three sites (Sites 1 to 3) of the Tsurushi Formation and correlated the radiolarian assemblages with the previously proposed radiolarian zones. At the Site 1, we discriminated Eucyrtidium inflatum. E. inflatum indicate the E. inflatum Zone, which corresponds to 15.3–11.7 Ma. At the Site 2, E. inflatum, Lychnocanoma kamtschatica and Lychnocanoma magnacornuta were identified. On the basis of the occurrences of these species, the formation of the Site 2 can be correlated with the E. inflatum Zone (15.3–11.7 Ma) to the L. magnacornuta Zone (11.7–9.1 Ma). L. kamtschatica has been reported only from the high latitudes of the North Pacific.The occurrence of this species from the Tsurushi Formation is its first occurrence from the Japan Sea region. At the Site 3, relatively a lot of specimens of Cyrtocapsella tetrapera and Cyrtocapsella japonica are identified, which indicate the Subzone a of the E. inflatum Zone or older zones.</p>

Radiolarian fossils from the Miocene Tsurushi Formation distributed in Sado Island, Niigata Prefecture, Japan

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