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Volume 108, Issue 3
Displaying 1-8 of 8 articles from this issue
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Hayato Ueda, Makoto Kawamura, Kohki Yoshida2002 Volume 108 Issue 3 Pages 133-152
Published: March 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSCretaceous forearc basin deposits of the Nitarachi-Yezo Sequence overlie the high-P/T metamorphosed Iwashimizu accretionary complex in the Mitsuishi River area of the southern Kamuikotan Zone. In the sequence, the Albian Utafue Formation (Middle Yezo Group) unconformably overlies the Valanginian-Barremian Iwashimizu Complex. The basal part of the Utafue Formation comprises a fluvial sequence, which contains greenstone clasts with high-P/T metamorphic minerals such as Na-amphibole and Na-pyroxene. The conglomerate also contains Valanginian-Barremian mudstone clasts derived from the Nitarachi-Yezo Sequence. The fluvial conglomerate clearly shows that the Iwashimizu Complex and lower part of the Nitarachi-Yezo Sequence were exposed subaerially in the Albian and were supplying clasts to the forearc. The Iwashimizu Complex, the high-grade part of which was metamorphosed at a depth of over 20km, is thus concluded to have been rapidly exhumed within ca. 20Ma after peak metamorphism. The complex was exhumed as a coherent mass, not as a serpentinite diapir. The exhumation is inferred to have resulted from lifting by underplating of abundant seamount volcanics, and by increased buoyancy of subducted oceanic lithosphere, which became younger during the Cretaceous.View full abstractDownload PDF (2816K) -
Masatsugu Sueoka, Akihiro Kano, Tomoko Kojo, Jun Matsuoka, Takuji Ihar ...2002 Volume 108 Issue 3 Pages 153-163
Published: March 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSTufas of southwestern Tokuno-shima Island in subtropical Ryukyus reveal some chemical and depositional characteristics which differ from those of previously-described tufa deposits from temperate regions in Japan. Warm climate reflects high soil Pco2 and the resultant high Ca contents of the ground water, and also rises water temperature of the stream water. All these conditions cause high inorganic precipitation rate of calcite. The rate evaluated by using the water chemistry data is nearly twice larger than the values recorded in the temperate regions. This is consistent to the observed large growth rate of tufa, up to 2.3cm/year. The largest evaluatedprecipitation rate was recorded in CO2-degassed water of a pool where the shrublike deposits are developed. The tufas of Tokuno-shima often exhibit laminated textures, but the lamination pattern is irregular and cannot be regarded as an annual banding. Probably some processes, as well as seasonal variation in climatic conditions, are related to banding of tufa deposits. The tufas of Tokuno-shima are various in terms of internal textures and associated cyanobacteria. However, these variations may be ascribed to variations in hydrology and associated biota due to the collecting locations, rather than the subtropical climatic conditions.View full abstractDownload PDF (1793K) -
Yoshiro Ishihara, Shuichi Tokuhashi2002 Volume 108 Issue 3 Pages 164-175
Published: March 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSTwo types of turbidites, i.e. shallow-marine or shelfal one in the east and deep-marine or submarine-fan one in the west, were concurrently formed and accumulated to form the Pliocene Kawaguchi Formation at the eastern margin of the Tertiary Niigata sedimentary basin, central Japan. Based on the paleocurrent and heavy-mineral analyses, these two types of turbidites were supplied westward from the different sources in the eastern mainland (Tokuhashi, 1992 a, 1996). As time-series fluctuation of turbidite deposition is controlled by allocyclic and autocyclic systems, the characteristics of the different depositional systems can be detected by numerical analysis. In the present paper, first, bed thickness frequency distributions and time-series fluctuations of turbidite successions from western deep-marine facies (4 section) and eastern shallow-marine facies (3 section) were analyzed. Then, sedimentary process and system of turbidites of the Kawaguchi Formation are discussed based on the results of these numerical analyses. The main results are as follows : (1) thickness frequency distributions of non-turbidite mudstone beds in western deep-marine facies have larger variance ; (2) time-series of both turbidite thickness and number of turbidite beds fluctuate discontinuously due to the existence of turbidite packets which are formed by dozens of turbidite bed sets during several tens of years ; (3) time-series fluctuation of the advent of turbidite packets in each sections of deep-marine facies varies in sync by the Milankovitch order cycles ; (4) time-series fluctuations in shallow-marine turbidites are consecutive and independent from deep-marine ones. These results suggest that the depositional system of the western deep-marine Kawaguchi Formation, comprised of discontinuous narrow depositional tongues, has a main feeder channel, which were detached and connected to the mouth of onshore main supply river by the influence of sea-level fluctuation. However, the eastern shallow-marine turbidites were not influenced by the same environmental change and deposited independently under the different sedimentary system.View full abstractDownload PDF (1486K) -
Akira Miyake, Yuka Onishi, Setsuo Yogo2002 Volume 108 Issue 3 Pages 176-185
Published: March 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSWe propose a new method for measuring c-axis orientations of optically uniaxial crystals with conoscopic figures. The fundamental operation in this method is that a crystal is rotated on the microscope stage under conoscopic illumination until extinction occurs at a test point specified in a conoscopic field of view and that the rotation position is measured on the microscope stage scale. This operation is done for two test points in order to obtain a c-axis orientation of the crystal. We set the two test points at positions equidistant from the conoscopic field center in both directions along the E-W hair. The distance from the field center to these test points can be connected with the wave normal direction within the crystal for the light passing each test point. When the rotation position satisfies the condition that extraordinary light along the wave normal vibrates at right angles to the E-W hair, the test point will be in extinction state. The extinction condition was formulated as equations in terms of c-axis orientation, distance from the field center to the test points, principal refractive indices of the crystal, and stage readings for the two test points in extinction state ; c-axis orientations can be calculated using the equations. The conoscopic method is superior to the conventional U-stage method in its accuracy (i.e. the measurement error in c-axis orientation could be less than 0.2° for quartz).View full abstractDownload PDF (1297K) -
Gentaro Kawakami, Masamichi Shiono, Makoto Kawamura, Akiko Urabe, Itar ...2002 Volume 108 Issue 3 Pages 186-200
Published: March 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Miocene Kawabata Formation consists of turbidites and associated coarse clastics, and shows an N-S trending elongated distribution in the western Yubari Mountains, central Hokkaido. The Kawabata Formation is divided into two members. The lower, Amagiri Sandstone and Mudstone Member is characterized by monotonously interbedded sandstone and mudstone with sporadically intercalated conglomerate beds. Pebbly mudstone is present at two horizons of this member. The upper, Higashiyama Sandstone and Conglomerate Member consists of thick-bedded conglomerate and sandstone with sand-rich alternation of sandstone and mudstone. Based on the correlation of five tuff beds (K1-K5, in ascending order), the lithostratigraphic boundary between these two members is younger in the south than the north. By the fission-track method, the age of tuff bed K5 is 13.2±0.9 Ma. Diatoms from beneath the K4 are indicative of the Crucidenticula nicobarica Zone to Denticulopsis praedimorpha Zone (13.1-11.5Ma). Diatoms from a horizon 400m above the K5 are indicative of the Denticulopsis dimorpha Zone (10.0-9.2Ma). The depositional age of Kawabata Formation thus ranges up to Late Miocene. The change in lithology from mud-rich alternation (the Amagiri Sandstone and Mudstone Melnber) to thick-bedded con-glomerate and sandstone (the Higashiyama Sandstone and Con-glomerate Member) was earlier in the northern area. Pebbly mudstone of the Amagiri Sandstone and Mudstone Member thickens northward. These suggest southward progradation of the basin-fill deposits. In contrast to the Miocene formations in northern Hokkaido and the Hidaka coastal area, gravelly deposits were continuously supplied until Late Miocene in the Yubari Mountains.View full abstractDownload PDF (2274K) -
Hironori Hiranaka, Shigeyoshi Matsubara, Katsuki Kurokawa2002 Volume 108 Issue 3 Pages 201-204
Published: March 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSCorrelation of late Miocene volcanic ash beds in the Uchisugawa and Nomura Formations composed of hemipelagic mudstone in Niigata Prefecture was attempted. Because glass shards in these ash beds are preserved from chemical alteration, their shape and chemical composition by EPMA analyses give useful criteria of tephra identification. Four volcanic ash beds distributed in each formation were well correlated ; namely (1) Kdg (in the Uchisugawa Formation) -Sng (in the Nomura Formation) beds, (2) Gtm-Stm beds, (3) Skhq-Tmhq beds, and (4) Jng-Snsg beds in ascending order.View full abstractDownload PDF (712K) -
Rei Nakashima, Kazutaka Amano, Vladimir D. Khudik2002 Volume 108 Issue 3 Pages V-VI
Published: 2002
Released on J-STAGE: December 14, 2010
JOURNAL FREE ACCESS -
Satoru Kojima, Chitaro Gozu, Tetsumaru Itaya, Talat Ahmad, Rafique Isl ...2002 Volume 108 Issue 3 Pages VII-VIII
Published: 2002
Released on J-STAGE: December 14, 2010
JOURNAL FREE ACCESS
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