The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 110, Issue 2
Displaying 1-7 of 7 articles from this issue
Articles
  • Takuro Shuto, Tsutomu Otsuka
    2004 Volume 110 Issue 2 Pages 67-84
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    The Misogawa Complex is a Mesozoic accretionary complex widely exposed in the eastern Mino Terrane, central Japan. The Misogawa Complex in the Kisofukushima area consists of two lithologic units; one is chert-dominant unit and the other is sandstone-dominant unit. The former is composed of pelagic chert and hemipelagic siliceous mudstone, and the latter is composed of a large amount of sandstone and mudstone. They show various deformation features up to dismembered formation. In the chert-dominant units, chert-clastics sequences that are composed of pelagic chert and overlying trench-fill sediments are repeatedly exposed with imbricate thrusts. The trench-arrival age of the oceanic plate is Late Jurassic and the depositional age of the complex possibly reaches latest Jurassic or earliest Cretaceous on the basis of radiolarian biostratigraphic data obtained from three continuous successions. The trench-arrival age shows the structurally downward younging polarity through the structurally overlying Sawando Complex and the underlying Misogawa Complex. It is inferred that both complexes were formed through continuous subduction-accretion process from late Early Jurassic to latest Jurassic or earliest Cretaceous. Considering lithofacies, structures and radiolarian ages, the Takatori Unit and the Kasama Unit in the Yamizo Mountains and southern part of the Kamiaso Unit in the western Mino Terrane are correlative with the Misogawa Complex.
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  • Hiroki Hayashi, Masaki Takahashi
    2004 Volume 110 Issue 2 Pages 85-92
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Late Miocene planktonic foraminiferal biostratigraphy was presented from the Arakawa Group deposited in the intra-arc basin of the forearc side, central Honshu, Japan. The stratigraphic interval between the upper part of the Ogane and Tanokura Formations of the group is correlated with the planktonic foraminferal Zone N.16 of Blow (1969) because of the occurrence of Neogloboquadrina acostaensis and the absence of Globorotalia plesiotumida. This correlation shows no significant contradiction with other biostratigraphic results, namely, diatom, radiolarian, and calcareous nannofossil based on the standard age correlation table of Saito (1999). The species composition of the present study significantly differs from that of the contemporaneous Kubota Formation in the eastern Tanagura area, about 50km northeast of the present area. It is supposed that the difference between the two areas would be influenced by an intensity of a warm current.
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  • Takuichiro Kuwabara
    2004 Volume 110 Issue 2 Pages 93-102
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    In the northeast Japan arc, a narrow linear-lowland runs along volcanic front. The Kamikita Plain occupies the northern end of this tectonic linear-lowland; and mainly consists of shielding marine-terraces and thick marine-sediments. Using the tephrochronology, the author identifies four marine terraces hereafter referred to as the Fukuromachi surface, Shichihyaku surface, Tengutai surface, and Takadate surface from higher (older) to lower (younger) in this plain. The terrace deposit of Fukuromachi surface is composed of the Ohira Formation and Fukuromachi Formation in ascending order. The Ohira Formation is subdivided into two parts; bay-environment sandy-fill (lower Ohira Formation) and shoreface-environment sandy- to gravelly-layer (upper Ohira Formation). Moreover, the Fukuromachi Formation is subdivided into two parts; bay-environment sandy-fill (lower Fukuromachi Formation) and shoreface-environment sandy-layer (upper Fukuromachi Formation). The above succession of sedimentary environments show that relative sea-level changed from regression to transgression to regression before the construction of Fukuromachi surface. The terrace deposits of Shichihyaku, Tengutai, and Takadate surfaces are defined as the Shichihyaku Formation, Tengutai Formation, and Takadate Formation, respectively. The Shichihyaku Formation is composed only of shoreface-environment sandy-layer; but the Tengutai and Takadate Formations are subdivided into two parts; bay-environment sandy- to muddy-fills (lower Tengutai Formation and lower Takadate Formation, respectively) and shoreface-environment sandy-layers (upper Tengutai Formation and upper Takadate Formation, respectively). Especially, both bay-environment lower-parts bury the drowned valleys. Each sedimentary environment of Tengutai and Takadate Formations therefore show that relative sea-level changed from transgression to regression. The relative transgression of Takadate Formation is correlated with the marine isotope stage 5e high-stand. Therefore, the relative high sea-levels of Tengutai and Fukuromachi Formations are correlated with the ages older than the marine isotope stage 5e, probably the middle Pleistocene.
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  • Hiroyuki Hoshi, Kenji Miwa, Yuu Kawakami
    2004 Volume 110 Issue 2 Pages 103-118
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Samples for paleomagnetic and rock-magnetic measurements were collected from 30 sites of the northern part of the Middle Miocene (∼14 Ma) Kumano Acidic Rocks (KARs) in the southeastern Kii Peninsula. Directions of high-temperature or high-coercivity (high-T-C) magnetization components, as well as those of low-temperature or low-coercivity (low-T-C) ones, were determined by stepwise demagnetization at one site of granite porphyry and four sites of felsic welded tuff. Detailed rock-magnetic experiments indicate that the magnetization components reside primarily in low-Ti titanomagnetite of pseudo-single-domain state and that the sites from which we were not able to isolate any stable components from natural remanent magnetization contain pyrrhotite. The low-T-C components are probably a thermoviscous remanent magnetization acquired at the time of intrusion of granite which occurred after emplacement of granite porphyry. The high-T-C components of granite porphyry and welded tuff are considered as a primary thermoremanent magnetization and a remagnetization, respectively, both acquired during emplacement of granite porphyry around 14 Ma. Five site-means of the high-T-C components give a reversed-polarity mean direction having a clockwise-deflected declination (229.5°) and a steeply-inclined inclination (−66.6°). This mean direction closely resembles that of the southern part of the KARs, suggesting simultaneous emplacement of both the northern and the southern parts. Similarly, it is identical with the direction of a large-scale felsic pyroclastic flow deposit occurring in the northern Kii Peninsula (Muro Pyroclastic Flow Deposit), leading to a possibility that the northern part of the KARs is the source for the deposit and correlative pyroclastic units. The clockwise-deflected direction would have been acquired in an extraordinary geomagnetic field during a polarity transition or excursion.
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Prompt Reports
  • Hiroshi Yamamoto, Masaru Terabayashi, Hiroyuki Oasa, Yoshiyuki Kaneko, ...
    2004 Volume 110 Issue 2 Pages 119-122
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Pelitic metamorphic rocks of the Ryoke Belt are distributed in the Rokuroshi area, southern part of Iwakuni district. Dark-brown biotite schist was locally silicified and decolorized to form milky-white “silicified domain”. Quartz veins were developed in both of the biotite schist and silicified domain. The veins in biotite schist are generally parallel to the schistosity and form boudinage due to ductile flow of the host rock. The veins in silicified domain are oblique to the schistosity with medium to high angles and have not undergone ductile deformation except for some schistosity-parallel veins. The mode of occurrence of these veins indicates that the silicified domain is much more competent than the biotite schist. Ductile deformation after the silicification was accommodated by viscous flow of biotite schist. The silicification probably results from dissolution-precipitation processes which may have raised pore pressure to cause hydraulic fracturing.
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  • Akihiko Suzuki, Kiwamu Okamoto, Tsumoru Sagayama, Jun’ichi Uchid ...
    2004 Volume 110 Issue 2 Pages 123-126
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Twelve molluscan species were identified from the middle Miocene in Kenbuchi-cho, northern Hokkaido, which is assigned to the Denticulopsis lauta Zone (NPD 4A, 15.9-14.9 Ma). The molluscan assemblage includes some warm-water taxa such as Glycymeris cisshuensis and Liracassis japonica. The occurrence of the warm-water taxa indicates that warm water mass prevailed in northern Hokkaido. According to the molluscan evidence, northern Hokkaido was in a temperate marine zone during the early middle Miocene.
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