The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 118, Issue 4
Displaying 1-6 of 6 articles from this issue
Articles
  • Gentaro Kawakami, Junko Komatsubara, Tsumoru Sagayama, Kenji Nishina, ...
    2012 Volume 118 Issue 4 Pages 191-206
    Published: April 15, 2012
    Released on J-STAGE: September 27, 2012
    JOURNAL FREE ACCESS
    The Ishikari coastal plain, Hokkaido, northern Japan, is underlain by latest Pleistocene to Holocene incised-valley fills of up to 70 m thick. To elucidate the detailed stratigraphy and sedimentary features of these fills, two boring cores (GS-HTB-1 at 43°10′50.88″N, 141°29′10.66″E; GS-HTB-2 at 43°10′50.64″N, 141°29′06.92″E) were drilled at Kawashimo, Tobetsu Town. Both sites were located along the axis of the Paleo-Ishikari River during the last glacial.
    The succession in the cores is divided into five lithostratigraphic units on the basis of sedimentary facies, diatom assemblages, and AMS radiocarbon ages: Unit 1 (55.0–50.7 m below the ground surface), Upper Pleistocene flood-plain deposits; Unit 2 (50.7–45.0 m), gravelly river deposits; Unit 3 (45.0–22.4 m), meandering river deposits; Unit 4 (22.4–8.6 m), inner-bay deposits; and Unit 5 (8.6–1.5 m), fluvial and salt marsh deposits. The regressive succession from the upper part of Unit 4 to Unit 5 suggests progradation of a bay-head delta.
    The physical properties of the deposits correspond to their sedimentary facies and stratigraphic positions. The pH and electric conductivity values of the muddy sediments are in good agreement with the interpreted depositional environments. The accumulation curve obtained from the cores provides information on the timing of periods of environmental change at the studied site during a Holocene transgression: (1) the onset of deposition in a meandering river system, succeeding a gravelly river system, occurred at about 11 cal kyr BP; (2) the inner-bay environment rapidly expanded to the study site during 8–9 cal kyr BP; and (3) progradation of the bay-head delta on the bay floor occurred at around 8 cal kyr BP.
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  • Tadamasa Ueki, Satoru Harayama
    2012 Volume 118 Issue 4 Pages 207-219
    Published: April 15, 2012
    Released on J-STAGE: September 27, 2012
    JOURNAL FREE ACCESS
    Late Cretaceous crystal-rich rhyolites and related granites are widely distributed in the Inner Zone of southwest Japan. A great deal has been written about these crystal-rich rhyolites; however, little attention has been paid to their crystal-rich character.
    We focus on the remarkable crystal-rich character of these rocks with the aim of determining the magmatic conditions (i.e., temperature and initial water content) under which the Late Cretaceous Kizaki Rhyolite, and the associated Aoki Granite, formed within the Nishina Mountains, in the northeast of the Northern Japan Alps. The Aoki Granite contains unaltered fayalite and pyroxenes that retain information regarding the nature of the magma from which they formed. The Kizaki Rhyolite consists of a rhyolitic welded tuff, and its essential constituent (fiamme) is characterized by a crystal-rich nature (50%—60% by vol.). The Aoki Granite is remarkable for its lithological variability, but may be classified into three units: (1) fayalite- and pyroxene-bearing porphyritic granite, (2) chilled margin granite porphyry, and (3) mafic magmatic enclave-bearing finegrained biotite granite. These units have similar chemical compositions, characterized by high FeO*/MgO and high contents of CaO, Na2O + K2O, Ba, Zr, La, Ce, and Nd.
    We analyzed the granite porphyry of the Aoki Granite using a geothermometer and the MELTS program, and results indicate that it was generated from hot and dry magma (850—900 °C, initial water content = 0.4%—1.0% by wt.). This also suggests that similar crystalrich rhyolites, widely distributed through the Inner Zone of Southwest Japan, were generated from similarly hot and dry magmas, and that such magmatism was vigorous during the Late Cretaceous at the eastern margin of the Eurasian continent.
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  • Michiharu Ikeda, Hideaki Goto, Hiroyuki Tsutsumi, Koji Tsuyuguchi, Yuk ...
    2012 Volume 118 Issue 4 Pages 220-235
    Published: April 15, 2012
    Released on J-STAGE: September 27, 2012
    JOURNAL FREE ACCESS
    The Median Tectonic Line active fault system (MTLAFS) is one of the longest active faults in Japan, with a length of 400 km, and extends from the Kii Peninsula to Kyushu. Previous studies have obtained a large amount of data on recent surface-faulting events along the MTLAFS; however, the quality and quantity of this data is spatially and temporally inconsistent. More precise data are required to better assess faulting behavior and the magnitude of inland earthquakes along the MTLAFS. To this end, we excavated trenches across the Iyo Fault, which is on the western section of the MTLAFS, and obtained new geological information on three recent faulting events. The most recent faulting event occurred after AD 1650, the penultimate event occurred between 2865 BC and 845 BC, and an earlier event ooccurred between 23440 yrs BP and 4850 BC. The interval between the two most recent movements of the Iyo Fault was more than 2000 yr. We compared paleoseismological data from the past 10000 yrs with data from the Hongu Fault, which runs parallel to the Iyo Fault, and found that the Hongu and Iyo faults may have ruptured together, and at the same time as the penultimate event on the Iyo Fault.
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Note
  • Kazuhiro Suzuki, Koichi Mimura
    2012 Volume 118 Issue 4 Pages 236-239
    Published: April 15, 2012
    Released on J-STAGE: September 27, 2012
    JOURNAL FREE ACCESS
    Conodont elements in limestone are generally collected from acid-insoluble residues by using heavy solutions (e.g., bromoform). To avoid the use of harmful heavy solutions, we have developed a new method of identifying conodont elements in limestone. Acid-insoluble residues were obtained by treating limestone samples with 10% acetic acid solution. The collected residues were then soaked in a 10% acetic acid solution containing 0.1 wt.% Alizarin Red S for 5 days. Apatite (conodont elements, pellet pieces, abiotic apatite minerals, etc.) in the residues was stained a color between orange and light red, enabling it to be easily distinguished from other minerals. This method enables conodont elements to be collected from limestone samples effectively and safely.
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  • Arito Sakaguchi, Hideki Mukoyoshi
    2012 Volume 118 Issue 4 Pages 240-244
    Published: April 15, 2012
    Released on J-STAGE: September 27, 2012
    JOURNAL FREE ACCESS
    Vitrinite reflectance geothermometer was originally developed for use in oil exploration, but it has various applications; e.g., estimating the depth of burial in sedimentary basins, analysis of the thermal structure of accretionary complexes, and detecting evidence of frictional heat in fault rocks. However, use of the vitrinite reflectance geothermometer is not widespread. This paper describes a modified apparatus that was specifically designed to analyze small vitrinite particles in accretionary complexes and fault rocks. The original apparatus was designed to illuminate large coal samples over a wide field of view (>100 μm), and measured reflectance with a high-voltage photomultiplier. A wide field of illumination is required for petrographic identification of vitrinite; however, reflections from other bright minerals around the small vitrinite particles generate a high level of background noise. The apparatus described here has a dual illumination system: a homogeneous beam illuminates a wide field of view to allow identification of vitrinite, and a concentrated spot beam of only 1.6 μm diameter is used to measure reflectance. The spot illumination system measures reflectance with a semiconductor photodiode sensor. The avalanche-type photodiode has a sensor cooling system, provides high-resolution, low-noise reflectance measurements, and is simpler to use than the photomultiplier.
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