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

Middle Miocene to Quaternary diatoms in the Nankai Trough and Japan Trench

During Leg 87 of the Deep Sea Drilling Project, eleven holes were drilled at Sites 582 and 583 in the Nankai Trough, off Shikoku, southwestern Honshu, and three holes at Site 584 in the Japan Trench, off northeastern Honshu, Japan. In the former area, a low-latitude diatom zone called the Pseudoeunotia doliolus Zone is recognized in thick upper Quaternary sediments, which yield rare but characteristic admixtures of marine planktonic, marine tychopelagic-tobenthic, and nonmarine diatoms. In the latter area, all the sediments recovered contain abundant to common diatoms, allowing recognition of 12 continuous diatom zones from upper Quaternary through lower middle Miocene. Three hiatuses occur in this area around the Pleistocene/Pliocene boundary and in the upper and middle Miocene. In addition, 19 modified diatom zones for a lower Miocene through upper Quaternary interval are proposed. These middle-to-highlatitude zones are numerically coded (NPD1-NPD12) and represent the entire North Pacific. The establishment of these zones is based primarily on Leg 87 data and other DSDP materials and partially on several Japanese subaerial sequences. Correlation of the new zonal framework with previously established frameworks is attempted by the evaluation of operational usefulness of previously used datums. Resting spores of Chaetoceros and its related forms are recorded with specific intent for the first time, and possible ramifications of its frequency variation are presented. Nine new species are proposed: Delphineis sheshukovae Akiba n. sp., Denticulopsis praelauta Akiba and Koizumi n. sp., Kisseleviella ezoensis Akiba n. sp., Nitzschia umaoiensis Akiba n. sp., Thalassiosira jouseae Akiba n. sp., T. praenidulus Akiba n. sp., T. sancettae Akiba n. sp., T. umaoiensis Akiba n. sp., and T. urahoroensis Akiba n. sp. Transfers of systematic positions of the following four taxa are also proposed: Delphineis simonsenii (Mertz) Akiba n. comb., Ikebea tenuis (Brun) Akiba n. comb., Thalassiosira delicata (Barron) Akiba n. stat., and Thalassiothrixrobusta (Schrader) Akiba n. comb