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Volume 106, Issue 9
Displaying 1-7 of 7 articles from this issue
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Hiroshi Ohkubo, Tokiyuki Sato, Mahito Watanabe2000 Volume 106 Issue 9 Pages 583-596
Published: September 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Yabuta Formation consists of the early to late Pliocene marine sediments distributed along the Nadaura coast, and the Junicho Formation includes late Pliocene to early Pleistocene marine sediments near Asahiyama hill, in northwestern part of Toyama Prefecture.We examined volcanic ash beds in the Yabuta and Junicho Formations by petrographical method and measuring chemical component of volcanic glass. Two pairs of the volcanic ash beds (MT2-T1, UN-T3) can be correlated between the Yabuta and Junicho Formations, and the middle-upper part of the Yabuta Formation can be correlated to the lower part of the Junicho Formation. Fossil diatoms from the lower part of the Junicho Formation showed that the boundary between the Neodenticula koizumii-Neodenticula kamtschatica Zone and the N. koizumii Zone is around the T3 volcanic ash bed that is widely traced in the Hokuriku Region. Calcareous nannofossil biostratigraphy of the lower part of the Junicho Formation indicates that Datum A (2.75 Ma) is demarcated about 4 meter below the T3 volcanic ash bed. A widespread tephra of the Habutaki I-MT2 ash layer is situated about 14 m below the Datum A in the Junicho Formation.View full abstractDownload PDF (2520K) -
Ryuji Asada, Kazue Tazaki2000 Volume 106 Issue 9 Pages 597-608
Published: September 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSBiomineralization processes of silica associated with colonization of unicellular red alga, Cyanidium caldorium in green biomats were examined in this study. The green biomats occur in Higashi Hot Spring at the Satsuma-Iwo Jima Island in Kagoshima Prefecture, Japan. The biomats consist mainly of C. caldarium cells under acidic condition (<pH 2). The colonies of C. caldarium cells gradually change into angular shape with concentration of silica, and simultaneously organic matter is leached out together with P and S. The XRD and electron diffraction pattern indicated that the concentrated silica transformed from amorphous silica into cristobalite. When the colony developed, the inside part decolorized and became transparent due to loss of chlorophyll. Natural cultivation examination of C. caldarium indicates that the development of the colony depend on solution pH, nutrients, silica-concentration, growth rate of the cells, and eco-system with bacterial condition, and play an important role in silica-biomineralization.View full abstractDownload PDF (4385K) -
Tomoaki Sumii2000 Volume 106 Issue 9 Pages 609-619
Published: September 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSK-Ar ages were determined for the Miocene Setouchi-vocanic rocks in the western Setouchi Inland Sea region, southwestern Japan. The Setouchi-vocanic rocks in this area are olivine andesite, pyroxene hornblende andesite and biotite dacite. The obtained ages of nine andesitic rocks range from 16 Ma to 12 Ma, and most of them concentrate between 15.5 Ma and 14 Ma. Detailed petrographic observation of the specimen suggests that the effect of alteration is negligible in the obtained ages. These ages are about two million years older than the those reported previously. They will be important constraints on discussions regarding the genesis of the Setouchi-vocanic rocks and Miocene tectonics around this area.View full abstractDownload PDF (1638K) -
Akira Ono2000 Volume 106 Issue 9 Pages 620-631
Published: September 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSModes of occurrences of metamorphic and granitic rocks of the Atokura Nappe are studied to elucidate the geological structure of the Atokura Nappe. The examined klippes are exposed in the Minano-machi and Yorii-Ogawa-machi areas of the north-eastern parts of the Kanto Mountains. Small geologic bodies of metamorphic and granitic rocks of less than 1 km in size are tectonic blocks in fault contact with sedimentary rocks and quartz diorite. Many tectonic blocks consist of smaller tectonic blocks which were sheared and weakly metamorphosed. Geologic bodies including many tectonic blocks are also bounded by faults. Hence, the Atokura Nappe consists of various types of tectonic blocks which were formed in a root zone by contraction tectonics before the Nappe formation.View full abstractDownload PDF (2172K) -
Yukio Takayanagi, Koshi Yamamoto, Setsuo Yogo, Mamoru Adachi2000 Volume 106 Issue 9 Pages 632-645
Published: September 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe depositional environment of Cretaceous bedded cherts and intercalated shales from the Fukura area, Kochi Prefecture, in the Shimanto Terrane is discussed on the basis of analyses of n-paraffins (C12∼C38), major elements and rare earth elements (REEs). Radiolarian biostratigraphy confirms that the bedded cherts had deposited during Barremian.N-paraffins in most shale partings are low in concentration and are composed mainly of short-chained (C14∼C18). The low abundance of long-chained n-paraffins with a strong odd-carbon number predominance suggests that the bedded cherts deposited in an environment with little contribution of terrigenous organic matter.Some samples, in spite of their sampling horizon, shows lower Al2O3/TiO2 ratios, suggesting that the bedded cherts deposited around a field of mafic volcanism with a large contribution of basaltic debris.Samples from the lower horizon have higher tot.Fe2O3/TiO2 and MnO/TiO2 ratios than those of average shales and marine sediments. In addition, these are characterized by a negative Ce anomaly. This suggests a large contribution of Fe-Mn precipitates from hydrothermal emanations, originating from mafic volcanism. Samples from the upper horizons have lower tot.Fe2O3/TiO2 and MnO/TiO2 ratios than those from the lower horizons and are close to those of average shales and marine sediments. Moreover, these samples show no or slight negative Ce anomaly. The upper horizon bedded cherts, therefore, deposited in an environment with little contribution of Fe-Mn precipitate from hydrothermal emanations. Accordingly, the hydrothermal effect observed in the bedded cherts under consideration is inferred to have weakened drastically in a short period of time (about 5 m. y.).View full abstractDownload PDF (1842K) -
Tatsuki Tsujimori, Katsuichi Nishina, Akira Ishiwatari, Tetsumaru Itay ...2000 Volume 106 Issue 9 Pages 646-649
Published: September 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Fuko Pass metacumulate is a tectonic block (4.5×1.5 km in size) within the Oeyama peridotite body in the Inner Zone of southwestern Japan. It was metamorphosed under the high-pressure and moderate-temperature condition to form a mineral assemblage hornblende+clinozoisite+kyanite+paragonite+albite+rutile. K-Ar ages were determined on hornblende separates from four epidote amphibolite samples, yielding 443-403 Ma. This suggests a Siluro-Ordovician subduction-related metamorphic event for the Fuko Pass metacumulate, which is distinct from the Late Paleozoic high-P/T type metamorphic events of the Renge metamorphic belt in the Inner Zone of southwestern Japan. The Fuko Pass metacumulate is correlated, in age, with the Early Paleozoic high-P/T type schists (445-402 Ma) of the Kurosegawa klippe in the Outer Zone.View full abstractDownload PDF (735K) -
Hisatada Akahane, Masae Ohmori, Shoji Fujii, Chinseki Igawa2000 Volume 106 Issue 9 Pages XVII-XVIII
Published: 2000
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESSDownload PDF (9342K)
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