The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 108, Issue 7
Displaying 1-8 of 8 articles from this issue
  • Noritaka Endo, Fujio Masuda, Tetsuya Sakai, Miwa Yokokawa
    2002 Volume 108 Issue 7 Pages 415-420
    Published: July 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Small-scale bed waves under unidirectional currents can be classified into two types; Type I well-developed bed waves formed under a flow with a certain velocity, and sand grains move by avalanche on the lee slope; Type II: immature bed waves with short wavelength and low height formed at the early stage after the flow velocity decreases from the upper-plane-bed-regime, and grains on the bed move as a sheet flow. Experiments demonstrated that Type II bed waves could be preserved in the presence of ambient sedimentation and resulted in small-scale cross-stratifications similar to Type I formed under a relatively slow flow. Grain fabric analyses showed that the dominant direction of long axes of grains in the deposit of Type II bed waves dipped upstream against the lee slope, unlike Type I deposits. It is possible to identify the type of small-scale cross-stratifications by its grain fabric. We found an ancient example of ripples categorized into Type II in a turbidite sequence of the Plio-Pleistocene Kakegawa Group, in Japan.
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  • Yukito Kurihara, Yukio Yanagisawa
    2002 Volume 108 Issue 7 Pages 421-432_2
    Published: July 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Pectinid-dominant molluscan assemblage is first reported from the upper middle Miocene Shimotezuna Formation of the Taga Group in the Takahagi district, Joban area, southern part of northeast Honshu, Japan. Stratigraphic horizon of this assemblage is correlative with the upper part of the Denticulopsis praedimorpha Zone (NPD 5B) of Yanagisawa and Akiba's (1998) diatom zonation, and is estimated to be between 12.0 Ma and 11.5 Ma in age. This assemblage is interpreted to be an indigenous shallow-water marine pebbly sand bottom community on the basis of lithology, generic composition and mode of fossil occurrence. Specific composition of the pectinids in this assemblage differs from those in the contemporaneous assemblages in the central part of northeast Honshu, by containing more relict elements from the latest early to early middle Miocene Kadonosawa Fauna and less characteristic elements of the middle middle to early late Miocene Older Shiobara-Yama Fauna. Such differences may have resulted from a remarkable latitudinal gradient of shallow marine climate in the Pacific coast of northeast Honshu due to the late middle Miocene climatic cooling.
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  • Kazue Tazaki, Naoko Nawatani, Yukie Kunimine, Toshikazu Morikawa, Tosh ...
    2002 Volume 108 Issue 7 Pages 435-452
    Published: July 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    In 1985, a dam with a discharge gate was built at Dashidaira at Kurobe River, Toyama Prefecture for the first time in Japan. The dam sediments were first flushed out in December, 1991. The sediments with bad smell, such as sludge, spread all over the downstream of Kurobe River. After the first discharge of the dam sediments, benthic fishes decreased year by year in Toyama Bay. In this study, the sediments in Dashidaira Dam Reservoir and Toyama Bay were collected in 1997, 2000 and 2001. The Toyama Bay sediments which were considered to have been affected by discharged dam sediments were collected on July 20, 2000, November 19, 2000, and March 3, 2001. The sediments in the dam reservoir and the bay were studied chemically, physically and mineralogically for comparative study. The quantity of clay minerals, mainly smectite, increase toward the discharge gate at Dashidaira Dam. The distribution of clay minerals approximately corresponded to distribution of the N, C, and S concentration in Toyama Bay. XRD data show similar clay mineral components and patterns between dam sediments and suspended particles from the seabed at the offing of Kurobe River mouth. The sediments in Dashidaira Dam Reservoir and Toyama Bay contain relatively high content of kaolin minerals associated with chlorite, vermiculite, smectite, and mica clay minerals compared with sediments of the other bays. Rainbow trouts in water with smectite suspension result clearly indicated the damage to the fish. In the highest concentration of smectite (lOg/l), all 7 rainbow trouts died in five hours. The fish's gills were deformed and dehydrated, suggesting influence of smectite particles adhered to the surface of the gills. Furthermore, SEM observation and EDX analysis of the flat fish's gill in Toyama Bay clearly showed the presence of particle-like thin films with organic materials sticking on the surface. The volume of discharged sediments from Dashidaira Dam Reservoir is clearly related to the decrease of annual haul of benthic fishes in Toyama Bay within the past nine years (1991-1999).
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  • Takato Takemura, Masanobu Oda
    2002 Volume 108 Issue 7 Pages 453-464
    Published: July 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Microcracks-related fabric, determined by a stereological method, is represented quantitatively by means of the crack tensor, and brittle failure of granitic rock (Inada granite) is discussed based on the crack tensor analysis. Orientation of stress-induced microcracks does not change much during brittle failure, depending primarily on the orientation of pre-existing microcracks in quartz grains. Inelastic volumetric strain at failure decreases with increasing confining pressure due to the fact that crack opening is controlled by the confining pressure. The crack density F0 at failure, the first invariant of crack tensor, is independent of the confining pressure. That is, brittle failure starts when F0 reaches a threshold value. After failure, rapid increase of the crack density takes place with increasing inelastic volumetric strain, and the most of stress-induced microcracks are grain boundary cracks around highly cracked quartz grains. It is suggested, based on the observation, that such grain boundary cracks play an essential role in the subsequent process of deformation such as fault development.
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  • Atsuyuki Inoue, Nobuhiro Kotake, Yasutomo Sakaniwa, Akira Imai
    2002 Volume 108 Issue 7 Pages 465-473
    Published: July 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    This article describes the mode of occurrence, mineral composition, and oxygen and carbon isotope compositions of veins that are ubiquitously observed in the Upper Pliocene Shirahama Formation of the Chikura Group at the southern end of Boso Peninsula, Japan. The veins are characterized by milky to gray white in color, 1 mm to 3cm in thickness, and N 20° W-N40°W in strike, and divided from the textural viewpoints into three types : linear-, twig-, and spider's web-types. Spatial distribution of the veins is restricted within the Shirahama Formation and they do not extend to the Shiramazu Formation conformably overlaying on the Shirahama Formation. The vein minerals consist of, calcite, apophyllite, heulandite (or clinoptilolite), erionite, analcime, natrolite, thomsonite, and/or chabazite. Heulandite and erionite also are common in the matrix of tuffs, sandstones, and conglomerates of the Shirahama Formation. δ13CPDB and δ18OSMOW values of calcite ranged from -39‰ to -5‰ and +24‰ to +29‰, respectively. In spite of the slightly higher δ13C values than the published values of carbonates associated with cold seepages, it is inferred from the relatively low δ13C values that HCO3- in the vein-forming solution was derived from the oxidation and/or thermal decomposition of methane in sediments. By assuming the two values, 0‰ (a seawater value) and -5‰ (a present, meteoric water value), for the δ18O of solution equilibrated with calcite and using an oxygen isotope geothermometer, the formation temperatures of veins were calculated to be from 1°C∼48°C. The estimated temperatures of zeolite-bearing veins are significantly higher than those of cold seepage carbonates reported previously. The veins observed in the Shirahama Formation should be mentioned as an example representative of the diversity of cold seepages in their origin, chemistry, and flow mechanism.
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  • Takuro Kazuka, Tanio Ito, Yoshiaki Aita
    2002 Volume 108 Issue 7 Pages 474-477
    Published: July 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Early to Middle Eocene radiolarian fossils were newly found from a red shale bed of the uppermost part of the Sarugawa Formation in the Hidaka foreland fold-and-thrust belt, Hokkaido. Considering previous and our results, the age of the Sarugawa Formation was confirmed to range from Turonian-Coniacian to Early-Middle Eocene. This provides fundamental data for the reconstruction of structural settings just before the initiation of growth of the Hidaka foreland fold-and-thrust belt.
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  • Nobuharu Hori, Koji Wakita
    2002 Volume 108 Issue 7 Pages 478-481
    Published: July 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Well-preserved Jurassic radiolarians were obtained from two samples of manganese carbonate nodules which were collected from the Northern Chichibu Belt in the Ino district, Kochi Prefecture, Shikoku. These nodules were yielded from siliceous mudstone blocks embedded in the melange matrix. One sample (GSJ R76692) contains 234 species including A rchicapsa? pachyderma Tan, Hexasaturnalis hexagonus (Yao), Transhsuum hisuikyoense (Isozaki et Matsuda), Unuma echinatus Ichikawa et Yao and Tricolocapsa plicarum Yao, and thus the geologic age is assigned to early Bajocian. The other sample (GSJ R76694) contains 59 species with few age diagnostic radiolarians. However, the geologic age is assigned to late Early Jurassic on the basis of the faunal composition.
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  • Aiming Lin, Bihong Fu, Jinaming Guo, Qingli Zeng, Guangming Dang, Wenq ...
    2002 Volume 108 Issue 7 Pages XV-XVI
    Published: 2002
    Released on J-STAGE: December 14, 2010
    JOURNAL FREE ACCESS
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