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 117, Issue 7
Displaying 1-4 of 4 articles from this issue
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Articles
  • Stratigraphy and correlation of tephras in the Lower Pleistocene Kiwada Formation and its correlative beds, Kanto, Central Japan
    Takehiko Suzuki, Masanori Murata
    2011 Volume 117 Issue 7 Pages 379-397
    Published: July 15, 2011
    Released on J-STAGE: December 10, 2011
    DOIhttps://doi.org/10.5575/geosoc.117.379
    JOURNAL FREE ACCESS
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    We describe the characteristics of tephras preserved in Lower Pleistocene marine sediment in the middle part of the Kiwada Formation (Kazusa Group) and its correlative beds in the Tama hills, Tokyo (from drillcore), the Boso Peninsula, and the Choshi area, Kanto, central Japan. The tephras were correlated based on their mineral contents, refractive indices of minerals, and the shapes, refractive indices, and chemical compositions (major and rare earth elements) of volcanic glass shards. We re-examined 22 tephras (in descending order: HM-Kd16, GS, NK, NB-Kd17, SG-Kd18, MT, Kd19, NG-YR, YU, PK, YM, KK, TN, Sr-Kmd, Ob4e-Kd23A, Ob4d-Kd23B, Ob4c-Kd24, Ob4b-6 to 4, Ob4b-1, and Omn-SK110), revising their definitions, correlations, and distributions. The biostratigraphy of the Choshi area, based on calcareous nannofossil datum 9—11, indicates the tephras range in age from 1.343 to 1.633 Ma. The accumulation rate of sediments during this period, in all four study areas, was 1.4—225 cm/kyr, depending on the sedimentary environment.
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  • Stratigraphy and detailed age of the Upper Cretaceous Himenoura Group in the eastern part of Amakusa-Kamishima Island, Kumamoto, Japan
    Yuki Kojo, Toshifumi Komatsu, Tadataka Iwamoto, Reishi Takashima, Osam ...
    2011 Volume 117 Issue 7 Pages 398-416
    Published: July 15, 2011
    Released on J-STAGE: December 10, 2011
    DOIhttps://doi.org/10.5575/geosoc.117.398
    JOURNAL FREE ACCESS
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    The Upper Cretaceous Himenoura Group is exposed in the eastern part of the Amakusa-Kamishima Island, Kumamoto, Japan, where it is divided into the Hinoshima and Amura Formations, in ascending order. Inoceramids, ammonoids, and microfossils (e.g., radiolarians and foraminifers) are abundant in this area. The Hinoshima Formation yields Dicarinella canaliculata and abundant Marginotruncana pseudolinneiana (foraminifers), Inoceramus amakusensis (inoceramid), and Texanites kawasakii (ammonoid), indicating a Santonian age. The Amura Formation yields a Campanian foraminifera assemblage composed mainly of Grobotruncana linneiana and G. cf. arca; these species are also abundant in the upper part of the Amura Formation. Therefore, the Santonian—Campanian boundary is probably situated in the lower part of the Amura Formation. In the Hinoshima and Amura Formations, the radiolarian assemblage is characterized by Amphipyndax stocki, Dictyomitra formosa, and D. koslovae, indicating the Santonian to Middle Campanian.
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  • High-resolution digital transmitting microscope with a large objective lens
    Arito Sakaguchi
    2011 Volume 117 Issue 7 Pages 417-421
    Published: July 15, 2011
    Released on J-STAGE: December 10, 2011
    DOIhttps://doi.org/10.5575/geosoc.117.417
    JOURNAL FREE ACCESS
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    Standard optical microscopes are based on diffraction imaging, and their resolution is limited to half the wavelength of the illuminating light. In contrast, scattered-light imaging depends on the sharpness of the optics and is not limited by the wavelength. This technique is used mainly for reflective dark-field observations. The combination of a scattered-light illumination system and sharp optics may improve the resolution of the transmitting microscope. It is well known that the large objective lens in a telescope improves the resolution by reducing the diffraction rings. The digital microscope, designed for industrial applications, has a large objective lens and a 16-bit high-gradation image-processing system, resulting in a sharper image. In this paper, a digital microscope is compared with a standard microscope using the same condenser lens in both cases, which produces scattered light. To amplify the scattered light, the light beam around the optical axis is masked in the condenser lens. Although the standard microscope uses an objective lens with a higher numerical aperture (NA = 0.92) than the digital microscope (NA = 0.82), the digital microscope achieved a higher resolution (over 2500 line-pairs/mm) than the standard microscope, even under dry conditions. This resolving power is greater than that predicted by classical optical theory.
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