The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 109 , Issue 4
Showing 1-5 articles out of 5 articles from the selected issue
  • Ritsuo Nomura
    2003 Volume 109 Issue 4 Pages 197-214
    Published: April 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    During the 20th century, many Japanese brackish lakes have been significantly changed by human activity such as construction of water barrier gates, dikes, and coastal embankments. In addition to these modifications, sea level changes have influenced brackish biotas. However, few reports have discussed artificial and natural interactions in these brackish lakes. To evaluate these artificial and natural influences, the environment of Lake Nakaumi has been reconstructed from the 1920s to the present using foraminiferal analysis. Four periods of environmental changes can be recognized by factor analysis: 1920s-'30s, 1940s-'50s, 1960s-'70s, and the 1980s to the present. These four periods show that both man-made and natural impacts are significant for brackish biotas. When both impacts combine, the changes became more significant. Originally less mobile lower waters of Lake Nakaumi became activated during the middle 1940s-early '50s, coinciding with the development of the Ammonia and Milliolinella assemblages, which are the most diversified biotas of the 20th century. This period of environmental change is associated with sea level rise in the Sea of Japan. A large-scale land reclamation project produced significant changes in Nakaumi waters from the middle to late 1970s on. Topographically, Lake Nakaumi is almost closed. It was further enclosed by artificial construction. This created stable lower waters below a well-developed halocline, permitting occupation of eutrophic Nakaumi Proper Water (NPW) as indicated by the development of the Trochammina hadai assemblage.
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  • Yukito Kurihara, Seiji Horiuchi, Yukio Yanagisawa
    2003 Volume 109 Issue 4 Pages 215-233
    Published: April 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Lithostratigraphy is revised for the Miocene marine sequence in the Iwadono Hills area. Saitama Prefecture, central Japan, and integrated diatom and calcareous nannofossil biostratigraphy is established for the lower part of the sequence. The Miocene sequence in this area is subdivided into the Kamikarako, Godo, Negishi(redefined), Shogunzawa(redefined). Hatoyama, and Imajuku Formations in ascending order. These formations are essentially conformable except for the boundary between the Kamikarako and Godo Formations, where the Godo Formation is considered to overlie the Kamikarako Formation unconformably. Important nine key tuff beds for the local correlation of the Negishi and Shogunzawa Formations are described. The main part of the Kamikarako Formation is assigned to the Denticulopsis lauta Zone (NPD 4A) within the framework of North Pacific diatom zonation, but the lower most part of the formation is correlative with the Crucienticula Kanayae Zone (NPD 3A). The Negishi and the lowermost Shogunzawa Formations are assigned to the basal part of the Denticulopsis praedimorpha Zone (NPD 5B). Calcareous nannofossil biostratigraphic analysis indicates that the lower oart of the Negishi Formation is assignable to the Zone CN 4 and that the upper Negishi and the lowermost Shogunzawa Formations are correlative with the Subzone CN 5a. On the basis of the microfossil biochronology, geologic age of the unconformity between the Kamikarako and Godo Formations is restricted to 15.1-15.4 Ma of Middle Miocene. This unconformity is correlatable to the widespread uncoformity recognized in a number of Miocene sequences in central and northeastern Japan.
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  • Atsushi Kamei, Tetsuichi Takagi
    2003 Volume 109 Issue 4 Pages 234-251
    Published: April 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    In the Funehiki area, Fukushima Prefecture of the Abukuma granitic terrain of NE Japan, intermediate to felsic granitic rocks and gabbroic rocks are complexly distributed. The gabbroic rocks occur as small xenoblocks (1 to 3 km) within the surrounding granitic rocks. Based on geological relations and petrography, the granitic rocks have been divided into older and younger types. The older type is composed of weakly foliated granodiorite to tonalite with euhedral to subhedral hornblende. In contrast, the younger type comprises massive granodiorite to granite with K-feldspar phenocrysts or minor muscovite. The older type is represented by Nagaya, Shikayama, and Ishimori bodies, and the younger type comprises Oishizawa, Miharu, and Hatsumori plutons based on their lithofacies and intrusive relationships. Although all the granitic plutons are geochemicaly classified as calcalkaline I-type granitoids in volcanic arcs, each body has a different compositional trend on the Harker diagram. The gabbroic rocks belong to the tholeiitic rock series and their chemical compositions do not define a continuous trend typical of granitic rocks in the Harker diagram. Though magnetic susceptibility of the granitic rocks show generally low values of 0.2 to 1.0 × 10-3 SI unit corresponding to ihnenite-series granites, the Ishimori and Oishizawa granites have higher values of 1.0 to 30.0 × 10-3 Si unit corresponding to magnetite-series and/or ilmeniteseries granites. The gabboic rocks generally show magnetic susceptibility of 30.0 to 60.0 × 10-3 SI unit. The above characteristics of individual plutons and the gabbroic rocks suggest that all of the granitic and gabbroic rocks formed from discrete magmas. To study the petrogenesis of the granitic rocks, we have calculated the chemical composition of trace elements in the gabbroic rocks using partial melting model. This calculation shows that the granitic melt was formed by melting of a basaltic rock simultaneously forming remaining solid phase of amphibole or garnet + plagioclase.
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  • Takeshi Nakajima, Tohru Danhara, Hideki Iwano, Tohru Yamashita
    2003 Volume 109 Issue 4 Pages 252-255
    Published: April 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Fission-track dating was carried out on zircon crystals in a pumice obtained from a tuffbed in the Katsurane Facies in the Tentokuji Formation at Hanekawa, Akita City. As a result, the first reliable radiometric age of 4.4±0.6 Ma (2σ) was obtained for the tuffbed. The result supports the correlation between the Katsurane Fades distributed on surface outcrops and that distributed in offshore sub-surface wells.
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  • Satoshi Nakano, Junji Akai, Asahiko Sugaki
    2003 Volume 109 Issue 4 Pages VII-VIII
    Published: 2003
    Released: December 14, 2010
    JOURNALS FREE ACCESS
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