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 116, Issue 8
Displaying 1-9 of 9 articles from this issue
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Articles
  • Geology and Sr isotope age of the Lower Miocene Isomatsu Formation, Tsugaru Peninsula, Aomori Prefecture, Japan
    Shin-ichi Kamikuri, Isao Motoyama, Takanori Nakano, Kenshiro Ogasawara
    2010 Volume 116 Issue 8 Pages 403-411
    Published: 2010
    Released on J-STAGE: December 03, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.403
    JOURNAL FREE ACCESS
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    The Lower Miocene Isomatsu Formation is exposed on the northwestern part of the Tsugaru Peninsula, northern Japan, where it strikes NW–SE. The formation is subdivided into lower, middle, and upper parts based on lithology. The lower part consists mainly of conglomerate that contains cobbles of altered andesite derived from the underlying Gongenzaki Formation. The middle part is characterized by debris flow deposits, including oyster beds that represent the initial stages of transgression in the Early Miocene. The upper part is predominantly gravely sandstone and mudstone containing mollusc fossils. The characteristics of the sedimentary facies and thickness of the Isomatsu Formation appear to indicate deposition under shallow marine conditions, filling a half-graben bounded by a NW–SE-striking fault.
    Strontium isotope analyses (87Sr/86Sr) were performed on an oyster shell from the middle part of the Isomatsu Formation, yielding a 87Sr/86Sr age of 20.4±0.2 Ma. Based on this age and previous fission-track and K-Ar ages obtained for the underlying and overlying formations, we conclude that the Isomatsu Formation was deposited during the late Early Miocene (20–17 Ma). This age represents the time of the initial stage of transgression and the earliest invasion of mollusks into the study area from the Pacific Ocean in the Neogene. This event coincides with the eruption of basalt in the northwest Japan Sea, indicating that the initial transgression in the Tsugaru area was associated with sea-floor spreading in the northern Japan Sea at about 20 Ma.
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  • A new Turonian (Late Cretaceous) species of Inoceramus (Bivalvia) from the Yezo Supergroup, Hokkaido, northern Japan
    Akinori Takahashi
    2010 Volume 116 Issue 8 Pages 412-417
    Published: 2010
    Released on J-STAGE: December 03, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.412
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    A new inoceramid species (Inoceramus nakagawensis sp. nov.) from the Upper Cretaceous Yezo Supergroup, Hokkaido, Japan, is reported and systematically described in detail. The shell sculpture of I. nakagawensis sp. nov. is characterized by very fine, distinct, and regular to somewhat irregular concentric rings. Its occurrence is probably restricted to the upper Lower to lower Middle Turonian, making it a short-ranging species. The present species would be closely allied to Inoceramus cuvieri Sowerby, 1814, from the middle to upper Turonian Stage of Far East Russia.
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  • Re-examination of the cause of the Paleocene-Eocene thermal maximum, based on global carbon-cycle modeling
    Kazutaka Yasukawa, Kentaro Nakamura, Yasuhiro Kato
    2010 Volume 116 Issue 8 Pages 418-436
    Published: 2010
    Released on J-STAGE: December 03, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.418
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    One of the most dramatic global warming events in the Earth’s history occurred at the Paleocene-Eocene boundary (ca. 55 Ma). During the Paleocene-Eocene thermal maximum (PETM), the global temperature increased by more than 4°C within a few thousand years, accompanied by an abrupt negative carbon isotope excursion (CIE) in both the marine and terrestrial environments. Although this excursion implies a massive and rapid addition of 13C-depleted carbon to the oceans and atmosphere, the source of the massive carbon injection during the PETM remains uncertain.
    To provide some constraints on the cause of the PETM, we re-examined the observed magnitude of the CIE, and then reconstructed the perturbation of the global carbon cycle during the PETM, using a simple one-box global carbon-cycle model.
    The eruption of the North Atlantic Igneous Province (NAIP) appears to be the most plausible candidate for a triggering mechanism of the PETM. Our model indicates that the CIE of –3‰ is best explained by inputs of 2,200 Gt-C of thermogenic methane, produced by NAIP volcanism, and 700–2,800 Gt-C of biogenic methane produced by the subsequent dissociation of seafloor gas hydrates within 10 kyr. The uncertainty in the mass of released biogenic methane is attributed mainly to high variability in atmospheric pCO2 reconstructed for the late Paleocene (ranging from <300 to >2,000 ppm). However, the global temperature rise calculated by our model is 1.4°C at most, and we cannot reconstruct a temperature anomaly exceeding 4°C.
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Short Article
Reports
  • Early Jurassic radiolarian fossils from mudstone of the Ashio Terrane in the Kambara Mountains, Niigata Prefecture, Japan
    Takayuki Uchino, Rie S. Hori
    2010 Volume 116 Issue 8 Pages 441-446
    Published: 2010
    Released on J-STAGE: December 03, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.441
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    Early Jurassic radiolarian faunae were extracted from mudstone samples collected at three localities (Se1, Ik1, and Jo2) in an accretionary complex within the Ashio Terrane in the Kambara Mountains, Niigata Prefecture, Japan. The faunae of both Se1 and Ik1 are dominated by Canoptum species, in association with two characteristic species belonging to Gorgansium (G. sp. A and G. sp. B) for Se1, and species belonging to Parahsuum, Lantus, and Helvetocapsa for Ik1. The fauna in Jo2 consists mainly of the species of Parvicingula, Zhamoidellum, and gen. et sp. indet. B.
    Based on a comparison of these radiolarians with those from North America, Europe, and Southwest Japan, the ages of the faunae from Se1, Ik1, and Jo2 are Hettangian, Late Pliensbachian, and Middle to Late Toarcian/Aalenian?, respectively.
    This is the first report of radiolarian fossils from terrigenous clastic rocks (mudstone) in the Kambara Mountains; thus, this result contributes to reconstructions of oceanic plate stratigraphy in the region and to correlating the area with other accretionary complexes in the Ashio Terrane.
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  • Chemical composition of whole-rocks samples and volcanic glasses of the Miocene Tamateyama and Sekibutsu Tuffs in the area surrounding the Nara Basin
    Hironao Shinjoe, Hideki Iwano, Yutaka Wada, Yuji Orihashi, Tomoaki Sum ...
    2010 Volume 116 Issue 8 Pages 447-452
    Published: 2010
    Released on J-STAGE: December 03, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.447
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    We used XRF and INAA methods to analyze the chemical composition of whole-rock and volcanic glass samples from the Tamateyama and Sekibutsu Tuffs in the area surrounding the Nara Basin. Both tuffs have rhyolitic compositions with SiO2=73.6-75.0 wt.%, similar to that of the Muro Pyroclastic Flow Deposits (Muro PFD). The REE patterns of the tuffs are characterized by light REE enrichment, a pronounced negative Eu anomaly, and relatively constant chondrite-normalized middle to heavy REE abundances. These features are similar to those of S-type felsic rocks of the Outer Zone of southwest Japan. Our new petrochemical data provide additional constraints on correlations of the Tamateyama Tuff, Sekibutsu Tuff, and Muro PFD, and indicate that these units were derived from a large felsic igneous body in the Outer Zone of the Kii Peninsula.
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  • Petrography of cuttings from spa borings and their tectonic setting upon the Kanto Plain at Shimohara, Tsukuba city and at Hanai, Noda city
    Hideo Takagi, Yoshiaki Takeda, Toru Ishii
    2010 Volume 116 Issue 8 Pages 453-457
    Published: 2010
    Released on J-STAGE: December 03, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.453
    JOURNAL FREE ACCESS
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    This report describes the subsurface geology, based on the petrography of rock cuttings taken from bore holes at spa sites, at Shimohara, Tsukuba City, Ibaraki Prefecture, and at Hanai, Noda City, Chiba Prefecture, both upon the Kanto Plain. The cutting samples from Tsukuba (four samples from 1241–1210 m depth) are hornfels, biotite-muscovite schist, and sillimanite-andalusite gneisses that resemble the Tsukuba metamorphic rocks. A single cutting sample from Noda (1295–1300 m depth) is mylonitized biotite granite that is similar in lithology to the Late Cretaceous Name Granite in the Shimonita area, Gunma Prefecture. These results provide new information on the eastern extension of the Ryoke Metamorphic Belt and the southern extension of the Tsukuba Metamorphic Belt. The occurrence of mylonite from Noda also provides important information in terms of constraining the eastern extension of the Median Tectonic Line.
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Erratum
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