The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 109 , Issue 7
Showing 1-6 articles out of 6 articles from the selected issue
  • Takashi Okamoto, Takeshi Matsunaga, Motowo Okada
    2003 Volume 109 Issue 7 Pages 363-382
    Published: July 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    This paper is an attempt to provide a precise geological time scale for paleobiologists conceraed with the Upper Cretaceous fossils of the Haboro area and its neighborhood in northwestern Hokkaido, Japan. The Upper Cretaceous deposits exposed in the upper reaches of the Haboro-gawa (river) consist of six formations within the groups in ascending order as follows: (1) Tenkaritoge Formation, mainly composed of mudstone; (2) Haborodake Formation, conglomerate; (3) Shirochi Formation, alternating beds of sandstone and mudstone; (4) Lower Haborogawa Formation, predominantly claystone and siltstone (5) Upper Haborogawa Formation, upward-coarsening sequence of siltstone to coarse-grained sandstone; (6) Nagareya Formation, upward-coarsening sequence of claystone to silty sandstone. The former three formations belong to the Middle Yezo Group, and the latter three to the Upper Yezo Group. In order to enable direct correlation among the adjacent areas, subdivision into a total of eleven lithostratigrafic units within the sequences from Tenkaritoge to Nagareya Formations is newly proposed for this area. Borders between the units do not cross "chronological planes" so far as we trace three key-marker beds of acidic tuff. The adequacy of this newly proposed framework in this area was also tested by occurrences of macro fossils. The distribution of both ammonites useful for international stage correlation and several inoceramids zonal index species is quite concordant with previous knowledge established in other areas. Several other ammonites, which previously have been regarded as relatively long-ranging species, are found here to occur within quite restricted ranges.
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  • Masaki Matsukawa, Kazuto Koarai, Sho Shionoya, Takuya Shinkai, Kosuke ...
    2003 Volume 109 Issue 7 Pages 383-398
    Published: July 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Late Mesozoic nonmarine deposits are distributed widely on the eastern Asian continent and bear various kind of fossils such as dinosaurs, amphibians, reptiles, fishes, mammals, bivalves, gastropods, insects, ostracods, conchostracans, terrestrial plants and others. 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. The Tetori Group is one of the main late Mesozoic terrestrial deposits in East Asia, and bears common taxa in the area. 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 that is a main area of exposure for the group. Based on geological correlation among these areas, the group consists of a coherent stratigraphic section throughout the whole area. Therefore, the group is divided into seven formations, the Ushimaru. Mitarai, Otaniyama, Kuwajima/Okurodani, Amagodani, Okura and Bessandani in ascending order. The geological map of the area is then revised to show the distribution of these formations. This means the discussion and evaluation of zoo and phyto fossil assemblages as chronological and ecological are indices are possible in the corrected stratigraphic framework. Thus, the westward-shifting of the Tetori basin is much more easily interpreted by thickness and paleoenvironmental analysis of the Tetori Group sequences.
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  • Akiko Urabe, Fumio Akiba, Koichi Hoyanagi
    2003 Volume 109 Issue 7 Pages 399-412_1
    Published: July 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Diatom biostratigraphy of the Neogene strata in the Akan area, eastern Hokkaido is established by this study. The Neogene strata in the study area have been subdivided into the Tonokita, Chichappu and Kotan Formations in ascending order. The diatom assemblages from the upper part of the Tonokita Formation correspond to the Denticulopsis lauta Zone (15.9-14.9 Ma). The lower part of the Chichappu Formation correlates to the Thalassiosira praefraga and T. fraga Zones (23.0-18.4 Ma). The middle part of the Chichappu Formation is correlative to the D. lauta (15.9-14.9Ma), D. hyaline (14.9-13.1Ma), Crucidenticula nicobarica (13.1-12.9Ma), D. praedimorpha (12.9-11.5Ma), and Thalassiosira yabei Zones (11.5-10.1Ma). The upper part of the Chichappu Formation corresponds to the Rouxia californica (7.6-6.4Ma) and Neodenticula kamtschatica Zones (6.4-3.9/3.5 Ma). The diatom assemblages from the Kotan Formation are correlative with the R. californica and N. kamtschatica Zones, or indicate younger than the age of the N. kamtschatica Zone. These results lead to the following conclusions: 1) The Chichappu Formation includes three different horizons with hiatuses between them. 2) The lower part of the Chichappu Formation is the lower Miocene and indicates older age than that of the Tonokita Formation, which has been considered to underlie it. 3) The middle part of the Chichappu Formation is correlative with the upper part of the Tonokita Formation. These formations are correlative with the middle Miocene. 4) The upper part of the Chichappu Formation is correlative with the Kotan Formation. These formations correlate to the uppermost Miocene to Pliocene. The Miocene stratigraphy in the Akan area should be revised and redefined, because the current lithostratigraphic division is inconsistent with the age of diatom biostratigraphy of this study.
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  • Akira Ono
    2003 Volume 109 Issue 7 Pages 414-419
    Published: July 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    The Yoshimi metamorphic rocks were mylonitized to various degrees. Fine-grained mylonites are often found in shear zones, which are partly overprinted cataclasis. Pelitic mylonites are mainly composed of muscovite, chlorite, clinozoisite, calcite, albite, quartz and opaque minerals. Mylonite foliations are paralell to bedding planes. The thickness of mylonites in the Hinatayama region ranges from 5cm to 1m. The K-Ar bulk rock age is measured for a highly mylonitized pelitic rock which was not deformed cataclastically deformation. The K-Ar age determined is 18.0Ma. Taking into consideration the Miocene tectonics in the northern Kanto Mountains, it is considered that the mylonitization took place during the clockwise rotation of the Kanto Mountains in the middle Miocene time.
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  • Keita Umetsu, Atsushi Matsuoka
    2003 Volume 109 Issue 7 Pages 420-423
    Published: July 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Fossil spores and pollen were obtained from the Tetori Group (Itoshiro Subgroup and Akaiwa Subgroup) for the first time. They are composed mainly of fern spores. Identified spores include Appendicisporites spp., Cicatricosisporites spp., Cyathidites spp., Deltoidospora sp., Osmundacidites sp. and Schizaeoisporites spp. Gymnosperm pollen Classopollis sp. was rarely found in the Akaiwa Subgroup. The spore and pollen assemblage from the Akaiwa Subgroup can be correlated to Hauterivian to Aptian assemblage from the Songliao Basin, China.
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  • Akihisa Motoki, Yuji Orihashi, Daiji Hirata, Miguel Jorge Haller, Taka ...
    2003 Volume 109 Issue 7 Pages XIII-XIV
    Published: 2003
    Released: December 14, 2010
    JOURNALS FREE ACCESS
    In southern Andes back-arc region, behind the South Volcanic Zone (SVZ, 33°S-46°S) and Austral Volcanic Zone (AVZ, 49°S-52°S), hundreds of monogenic basaltic volcanoes take place (Fig.1), forming numerous cinder cones and lava flows (e. g. Ramos et al, 1982; Ramos and Kay, 1992). The eruption age ranges from the Oligocene to the Recent. The lava flows cover top of the sedimentary plateaux, forming table mountains, so-called "meseta" (Fig.2). In many cases, the plateau top lava is composed of only one cooling unit, less than 10 m total thick. Accumulated lava flows are also observed, and they are generally made up of less than four flow units of as much as of 30 m total thick. Some young lava flows fill glacial valleys, showing lava cascade (Fig.3). Certain lava flows and scoria falls layers are very fresh (Fig.4).
    Weathering, wind erosion, and glacial denudation expose cross section of the cinder cones and their underground structure, which makes possible the elaboration of their general structure (Fig.5). Young cones show fresh volcanic morphology almost without degradation (Fig.5A). Old cones loose surface non-welded scoria blanket, and remain their ba-cal core part made up of welded scoria and spatter (Fig.5B). Glacial erosion sometimes shows vertical section of the cinder cones (Fig.5C). Advanced denudation exposes silllike subvolcanic intrusive rock bodies (Fig.5C, 5D) and feeder dykes (Fig.5E).
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