The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 109 , Issue 8
Showing 1-7 articles out of 7 articles from the selected issue
  • Toshiyuki Kurihara
    2003 Volume 109 Issue 8 Pages 425-441
    Published: August 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Middle Paleozoic tuffaceous clastic and carbonate rocks are well developed in the Kuzuryu Lake-Upper Ise River area of the Hida-gaien Terrane in central Japan and are lithologically subdivided into the Kagero, Shibasudani, and Kamianama formations. The Kagero Formation consists mainly of tuffaceous sandstone and tuffaceous shale, which sometimes alternate, and differs from the other Paleozoic formations by having well-stratified tuffaceous rocks and an overall abundance of calcareous grains. The Shibasudani Formation is subdivided into three parts. The lower and upper parts consist mainly of sandstone and shale, with alternating sandstone and shale containing siliceous shale intercalations. The middle part of this formation is characterized by limestone blocks that are inferred to be exotic inclusions. The Kamianama Formation, which is correlative with the Lower Devonian Fukuji Formation, consists of limestone, felsic tuff, and sandstone. Radiolarians clearly indicate that the Kagero and Shibasudani formations are early Early Silurian and Late Silurian to Early Devonian in age, respectively. Terrane-wide correlations clearly show that the Middle Paleozoic strata of the Kuzuryu Lake-Upper Ise River area strongly resemble those of the Fukuji-Hitoegane area in age, lithology, and assemblage of lithologic units, and just as strongly differ from strata in the Hongo-Arakigawa and Naradani areas. Both Lower Devonian shallow-marine carbonates and deep-water tuffaceous elastics are present in the study area. Their occurrence together implies the presence of a diverse sedimentary system in a differentiated Middle Paleozoic basin.
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  • Yuka Hirahara, Kenji Shuto
    2003 Volume 109 Issue 8 Pages 442-458
    Published: August 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    The outcrop of the Cape Sukoton intrusion, a constituent of Miocene doleritic sill swarm in the northern part of Rebun Island, north Hokkaido, is 1, 600 × 600m wide and more than 240m thick, and dips gently eastward. It can be lithologically divided into three zones; the upper Columnar Joint Zone, middle Banded Structure Zone, and lower Massive Zone. The appearance of columnar joints is remarkable in doleritic rocks of the Columnar Joint Zone. Banded structures and columnar joints are also well developed in the doleritic rocks of the Banded Structure Zone, whereas both banded structures and columnar joints are not observed in porphyrites of the Massive Zone. Based on structural analysis of columnar joints, lithological descriptions of rocks in each zone, and the presence of interlayered sillsediment structure at the boundary between the intrusion and sedimentary rocks, the Cape Sukoton intrusion is infered to have formed in the following main three stage magmatic events. 1) first stage (formation of the Columnar Joint Zone); intrusion of a basaltic magma into wet and poorly consolidated sediments, and melanocratic to leucocratic dolerites and leucocratic andesitic veins were generated during subsequent cooling contraction and crystallization of magma, and columnar joints were formed throughout the body, 2) second stage (formation of the Banded Structure Zone); intrusion of a new basaltic magma into the Columnar Joint Zone and wet poorly consolidated sediments. During cooling of the magma, banded structures composed ostratified vesicular and non-vesicular layers have been formed in melanocratic dolerites, and 3) third stage (formation of the Massive Zone); intrusion of another magma into the Banded Structure Zone, prior to the complete cooling of the Banded Structure Zone. During the third stage, melanocratic dolerites of the Banded Structure Zone might have been brought into the intruding magma, which resulted in the appearance of melanocratic enclaves contained in the Massive Zone.
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  • Hirokazu Ozawa
    2003 Volume 109 Issue 8 Pages 459-465
    Published: August 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    The past work regarding the distribution of three representative fossil cold-water ostracod genera in surface sediment of the continental shelf along the southwestern to eastern margin of the Japan Sea is reviewed and summarized. The highest number of cold-water specimens is from the lower continental shelf off Yamaguchi Prefecture, northeast of the Tsushima Strait in the southwestern part of the Japan Sea. These fossils are considered to be the evidence of the southward expansion of their distributional limit during the postmiddle Pleistocene glacial interval, to a latitudinal limit of approximately 10° farther south than the Recent distributional limit of these genera. The presence of these fossil ostracods indicates the prevalence of water similar to the Japan Sea Central Water as far south as the northeast of the Tsushima Strait during the glacial period. This suggests that the summertime water temperature on the upper continental shelf during the glacial period was approximately 10°C lower and salinity 0.5‰ lower than Recent temperatures and salinities. The ostracod data suggest that a cold-water mass different from the Oyashio water was present in the southwestern Japan Sea during the post-middle Pleistocene glacial period.
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  • Masaki Matsukawa, Kazuto Koarai, Atsushi Okubo, Makoto Ito
    2003 Volume 109 Issue 8 Pages 466-477
    Published: August 15, 2003
    Released: April 11, 2008
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    The Tetori Group is one of the more important Mesozoic terrestrial deposits in East Asia, with taxa common throughout the area. Biostratigraphic resolution of these non-marine species as chronological indices is a moot problem which, to be answered adequately, requires thorough paleoecological and paleoenvironmental analysis. Hitherto, regional formation names of the Tetori Group were given separately for sequences in the Shokawa, Oshirakawa, Shiramine and Takinamigawa areas of the greater Mount Hakusan region which can be considered the main "type" section for the group. Based on geological correlation between these separate areas, the group was shown to consist of a coherent stratigraphic sequence thoughout the whole region and the geological map was revised to reveal the correct distribution of these formations, thus, we can discuss and evaluate zoo- and phyto- fossil assemblages as chronological and ecological indices and as geologically meaningful framework. Terrestrial vertebrates and freshwater bivalves occur mainly in the Kuwajima and Okurodani formations, deposited in flood plain and lake environments. The rich terrestrial plant fossils are also obtained from the Kuwajima and Okurodani formations. The very limited occurrence of these fossils in others fluvial formations, suggests that these appearances are controlled by environmental factors. There are two types of zoo-assemblages in the Kuwajima and its coeval formations; vertebrate species whose ancestors flourished in the Jurassic and their descendants in the late Cretaceous periods. As the latter is more superior in numbers than the former, changeable environments at the continental margin probably stimulated adaptation of the later species.
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  • Koji Kameo, Keiji Saito, Nobuhiro Kotake, Makoto Okada
    2003 Volume 109 Issue 8 Pages 478-488
    Published: August 15, 2003
    Released: April 11, 2008
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    Qualitative and quantitative analyses of nannofossil assemblages from the Mera and Minamiasai Formations, distributed in the southernmost part of the Boso Peninsula, were performed to determine geologic ages of both formations and to charify environmental changes in surface waters related to climatic fluctuations around central Japan during the late Pliocene. Six upper Pliocene nannofossil datums were detected in the Mera and Minamiasai Formations and both formations are assigned to 3.3 ∼ 2.4 Ma. Paleoenvironmental analysis was made by division of nannofossils into ecological groups based on their temperature preference and trophic regime (oligotrophic vs eutrophic). Between 3.3 and 3.0 Ma, the existence of the warm and oligotrophic surface water was suggested in the central part of the Pacific side of Japan because of abundant occurrences of Discoaster spp. And Florisphaera profunda. It means that the surface water was more oligotrophic as a result of spreading open ocean waters. After an decrease of warm water taxon at 3.0 Ma, strong influence of cool and eutrophic waters appeared in the studied region based on increases of very small Reticulofenestra and Coccolithus pelagicus. This indicates that the mixed water moved southward and its effect became stronger associated with high-latitude cooling at approximately 2.7 Ma. At the same time, inflow of nutrient-rich surface water originated from nearshore waters around the East Asian Continent also increased because of abundant occurrence of Helicosphaera, suggesting intensified weathering of the East Asian Continent related to tectonic processes around the Himalayan-Tibetan Complex.
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  • Hisashi Suzuki, Kiyoko Kuwahara
    2003 Volume 109 Issue 8 Pages 489-491_1
    Published: August 15, 2003
    Released: April 11, 2008
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    Late Permian (early Kuman) radiolarians are reported and depicted from Sado Island of Niigata Prefecture, central Japan, for the first time. The radiolarian fauna is obtained from mudstone. It is composed of Follicucullus charveti Caridroit et De Wever, FoUicucullus ventricosus Ormiston et Babcock, Entactinosphaera aff. Cimelia Nazarov et Ormiston, Entactinosphaera pseudocimelia Sashida et Tonishi, Hegleria mammilla (Sheng et Wang) and so on. A part of the pre-Tertiary rocks of Sado Island should be correlated to the Permian of the Ultra-Tanba Belt or Joetsu Belt, although they have previously been regarded as one of the Mesozoic accretionary complexes of the Ashio Belt.
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  • Katsuyoshi Michibayashi, Bunji Tojo, Ryo Saito, Shin-ichi Kawakami, To ...
    2003 Volume 109 Issue 8 Pages XV-XVI
    Published: 2003
    Released: December 14, 2010
    JOURNALS FREE ACCESS
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