Oligocene-Early Miocene Molluscs and Diatoms from the Kitami-Tsubetsu area, Eastern Hokkaido, Japan

Transitional upper Oligocene-lower Miocene marine deposits crop out in the Kitami-Tsubetsu area of eastern Hokkaido and consist of the Tatsukobu and Tsubetsu Formations in ascending order. The Tatsukobu Formation (870-970 m thick) consists mainly of hard shale and siltstone and contains two upward fining sedimentary cycles : the Sandstone to Lower Hard Shale members and the Sandy Siltstone to Upper Hard Shale members. The Tsubetsu Formation (950-1, 100 m thick) conformably overlies the Tatsukobu Formation, is mainly composed of massive siltstone and contains well bioturbated and glauconitic sandstone in its basal part. Overall, one sedimentary cycle is recognized in the Tsubetsu Formation : Fine or Conglomeratic Sandstone, Hard Shale to Siltstone members with an upward coarsening in the uppermost sandy part of the last member. Diatom zonal subdivision and geologic age assignments of the two formations are based on the scheme of Gladenkov and Barron (1995) established at the Detroit Seamount, off eastern Kamchatka. The main parts of the Tatsukobu and Tsubetsu Formations are assigned to the upper upper Oligocene Rocella gelida Zone and the upper lower Miocene Thalassiosira fraga Zone, respectively, while the uppermost part of the Tatsukobu and the lowermost part of the Tsubetsu beds appear to be assignable to the uppermost upper Oligocene to lower lower Miocene T. praefraga Zone. The Oligocene/Miocene boundary, therefore, lies somewhere near the boundary between the two formations. Nonmarine planktonic diatoms represented solely by Aulacoseira species are restricted to the T. fraga Zone, which suggests a significant change of depositional environment at that horizon. Odontella sawamurae n. sp. is one of the most characteristic diatoms of the Tatsukobu Formation. There are four assemblages in the Tatsukobu molluscan fauna : Bathymalletia-Nuculana (Borissia)-Acilana, Nuculana (Borissia)-Portlandia (Portlandella)-Periploma, Macoma-Lucinoma-Periploma and Mytilus-Septifer-Macoma. The Tsubetsu molluscan fauna contains six assemblages : Acilana-Megayoldia Nuculana (Nuculana), Limopsis, Macoma-Periploma, Macoma-Mya and Zirphaea. Most of the assemblages indicate rather deep-water, mesoneritic to bathyal environments during nearly all the time of deposition of both the formations through the geographic area studied. Based on lithology and the distribution of molluscan assemblages, relatively shallower-water conditions are interpreted for the southern part during these times. Both the Tatsukobu and Tsubetsu molluscan faunas include assemblages with Acilana tokunagai and may be assigned to the so-called "Acilana tokunagai fauna." The latter is considered to be several widely distributed correlative assemblages which characterized bathyal muddy environments in the northwestern Pacific during the Oligocene-middle (? upper) Miocene. Four new molluscan species are described : Limopsis tsubetsuensis, Septifer kitamiensis, Megacardita(?) tatsukobuensis and Zirphaea tsubetsuensis. The Tatsukobu molluscan fauna is comparable taxonomically to the well known late Eocene-Oligocene Asagai-Poronai molluscan fauna, but includes some species more characteristic of the Miocene. The Tsubetsu molluscan fauna is biostratigraphically successive to the Tatsukobu fauna and contains species in common with deep-water associations of the early Miocene Akeyo-Sankebetsu and the early to middle Miocene Kurosedani-Kadonosawa-Chikubetsu molluscan faunas. Most species from the Tatsukobu and Tsubetsu Formations also occur in Sakhalin, Kamchatka and the Koryak Upland