The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 122, Issue 2
Displaying 1-5 of 5 articles from this issue
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Articles
  • Formation of Pisoliths at Hot Springs in Saturnia, Toscana, Italy
    Kazue Tazaki, Fumie Tazaki, Masayuki Okuno, Teruaki Takehara, Yasuhito ...
    2016 Volume 122 Issue 2 Pages 45-60
    Published: February 15, 2016
    Released on J-STAGE: June 17, 2016
    DOIhttps://doi.org/10.5575/geosoc.2015.0038
    JOURNAL FREE ACCESS
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    A well-known terrace-forming hot spring is located at Saturnia in the Maremma area of southern Toscana, Italy. The waters are circumneutral (pH around 7), mesophilic (around 37°C), and give off a strong sulfurous odor. Pisoliths, brown limestone, and green microbial mats are found in the area. Hot spring structures and compositions are determined based on mineralogical and chemical data obtained with a powder X-ray diffractometer (XRD) and an X-ray fluorescence analyzer (XRF). Microbial parameters are determined on sub-millimeter scales using a scanning electron microscope, equipped with an energy-dispersive X-ray spectrometer (SEM-EDS).
    XRD results indicate that the pisoliths are composed of calcite, native sulfur, and quartz, whereas the brown limestone contains mica, native sulfur, chabazite, and 7 Å clay minerals. XRF analysis indicates that the pisoliths contain mainly C, O, Ca, S, Si, and Sr, whereas the brown limestone contains high concentrations of O, S, Al, K, Ca, Fe, and Na. Because the pisoliths are Ca-rich, concentrations of heavy metals (Sr, Sn, and Pb) at the aqueous interface can be explained by combining XRF chemistry, XRD mineralogy, and SEM-EDS observations of green microbial mats. SEM-EDS elemental maps of the pisolith indicate the presence of apatite and framboidal pyrite crystals.
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  • Meso-scale brittle deformation structures in and around the Ikoma fault zone
    Keisuke Mitamura, Takamoto Okudaira, Muneki Mitamura
    2016 Volume 122 Issue 2 Pages 61-74
    Published: February 15, 2016
    Released on J-STAGE: June 17, 2016
    DOIhttps://doi.org/10.5575/geosoc.2015.0039
    JOURNAL FREE ACCESS
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    We report the results of field observations and microstructural analyses of meso-scale faults that cut Cretaceous granitoids along and near lineaments of the Ikoma active fault zone. Based on measurements of the slip direction on fault planes, a paleostress of N-S extension is determined using the multiple inverse method. This stress field differs from the present-day regional stress field of predominant E-W compression, inferred from the seismic data inversion and the multiple inverse method applied to the active faults. Illite crystals of various sizes collected from a fault gouge, give K-Ar ages of 45.2±1.0 Ma for the 0.2-0.4 µm fraction and 46.0±1.1 Ma for the 0.4-1.0 µm fraction. High- and low-temperature components of illite are quantified by decomposing X-ray diffraction patterns to extrapolate the authigenic and detrital end-members. The ratios of low-temperature illite (1Md) to high-temperature illite (2M1 and 1M) from a fault gouge are indistinguishable from one another. Based on the relative K-Ar ages of illite composites and of biotites from the host granitoid, the upper age limit of fault gouge formation is ca. 30 Ma. These results suggest that formation of the fault gouge occurred between ~45 and 30 Ma, indicating that the meso-scale faults in basement rocks formed by N-S extension during the late Eocene to early Oligocene, rather than active tectonics under E-W compression.
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  • Well-preserved diatoms (Bacillariophyta) from the lower Oligocene Nuibetsu Formation in the Urahoro area, eastern Hokkaido
    Chieko Shimada, Makoto Yamasaki, Kazutaka Amano, Robert G. Jenkins, Y ...
    2016 Volume 122 Issue 2 Pages 81-85
    Published: February 15, 2016
    Released on J-STAGE: June 17, 2016
    DOIhttps://doi.org/10.5575/geosoc.2015.0040
    JOURNAL FREE ACCESS
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    To provide insight into the biostratigraphy and evolutionary radiation of marine diatoms, Paleogene fossil records are essential because modern floral assemblages are thought to have originated in the late Oligocene. However, Paleogene diatoms are rarely reported from the Northwest Pacific Ocean because of their poor preservation and limited occurrence. In this paper, we report well-preserved diatoms from methane-seepage-induced limestone of the lower Oligocene (Rupelian) Nuibetsu Formation in the Urahoro area, eastern Hokkaido. The diatoms are dominated by Stephanopyxis spp., Odontella sawamurae, Hemiaulus spp., and resting spores. The assemblage is remarkable because it lacks late Oligocene to early Miocene index species, reflecting floral turnover during the Oligocene.
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