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Volume 108, Issue 5
Displaying 1-7 of 7 articles from this issue
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Akinori Takahashi, Tokuji Mitsugi2002Volume 108Issue 5 Pages 281-290
Published: May 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSChannel-fill deposits in the Saku Formation of the Teshionakagawa area, northern Hokkaido, are correlated to the Upper Turonian. Eubostrychoceras cf. japonicum, Inoceramus cf. pedalionoides and I. pictus minus occurred from gravels filling the channel structure. The former two species co-occurred in a single gravel. We suggest the following three interpretations based on these results and the ages suggested for E. japonicum (Middle to Late Turonian) and I. pictus minus (Late Cenomanian) by previous studies:(1) The gravels containing I. pictus minus demonstrate that the channel structure scored as deep as the Cenomanian in the upper reaches, or the large-scale regression in this area denuded the underlying Cenomanian in the land and/or the shallow shelf area;(2) the first occurrence of I. pedalionoides may date back to the early part of the Late Turonian-Middle Turonian ; and (3) the calcareous concretion (gravel) containing E. cf. japonicum should have been consolidated within the time between the first occurrence of E. japonicum (Middle Turonian) and the sedimentation of the channel-fill deposits (Late Turonian).View full abstractDownload PDF (2328K) -
Ken-ichi Kano, Tadashi Maruyama, Aiming Lin2002Volume 108Issue 5 Pages 291-305
Published: May 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSLate Quaternary activity of the NW-SE trending Sakaitoge Fault in the southern Hida Mountains, Central Japan, has been evaluated based on the observations of tectonic landforms and fault outcrops. A few hundred meters left-lateral displacement is estimated by the offset streams along the fault those erode the Middle-Upper Pleistocene terraces. The fault also displaces vertically the terraces from a few meters to a few tens of meters. The asymmetrical fabrics of cataclastic fault rocks developed in the basement rocks show that the fault has a major left-lateral strike-slip displacement with minor vertical components. These lines of evidence clearly suggest that the fault has a slip-rate of several millimeters per year since the Late Pleistocene. The 14C datings of displaced sediments give the age of the latest faulting event after 1400±40 y.B.P. that was probably associated with the 762 A.D. or 841 A.D. earthquake.View full abstractDownload PDF (4530K) -
Ichiko Shimizu, Koji Shimada2002Volume 108Issue 5 Pages 306-317
Published: May 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSA new method for quantitative analysis of rock texture using a laser scanning microscope (LSM) is proposed. We developed a polarized LSM to identify constituent minerals and analyze their optical properties. High resolution digital images of rock textures are obtained by our LSM system. Confocal images of rock specimens under reflected laser beams a used clearly to visualize grain boundaries, and further analyze the texture by the aid of digital image processing techniques. Polarized images of rock textures under blue (B), green (G), and red (R) laser beams can be superimposed to construct color (RGB) microscopic images. Optical properties of minerals such as reflectance and retardation are accurately measured by the LSM system. Spectroscopic imaging of rock texture is used to visualize spatial distribution of constituent minerals.View full abstractDownload PDF (2653K) -
Takashi Oshimizu, Yasufumi Iryu2002Volume 108Issue 5 Pages 318-335
Published: May 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Pleistocene Ryukyu Group composed of reef complex deposits crops out on six islands off Katsuren Peninsula, Okinawa-jima, Ryukyu Islands, southwestern Japan. It conformably overlies Plio-Pleistocene sandy limestone and calcareous mudstone of the Chinen Formation containing abundant bioclasts and rests unconformably on Pliocene siltstone of the Shimajiri Group. We propose a major revision of the previous stratigraphic scheme for the Ryukyu Group and provide a formal stratigraphic description. The Ryukyu Group comprises the Yokatsu and Minatogawa Formations, in ascending order, on these islands. The Yokatsu Formation constitutes the main body of the group exceeding 30m in thickness and is exposed at elevations up to 125m. It is divisible into five units. Each unit comprises coral limestone and overlying rhodolith, Cycloclypeus-Operculina, and detrital limestones, showing a deepening-upward sequence. The Minatogawa Formation rests unconformably on the Yokatsu Formation, reaches 30m in thickness, and consists of well-sorted detrital and coral limestones that are thought to have been deposited in a shallow lagoon (moat);its surface exposure is confined to the areas at elevations less than ca. 40m in Ikei-jima and Tsuken-jima. Geologic ages of the limestones on these islands are unknown at this time.View full abstractDownload PDF (3361K) -
Akira Ono2002Volume 108Issue 5 Pages 336-346
Published: May 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSTectonics of mid-Cretaceous metamorphic belts, the Ryoke, Higo, Abukuma, Sanbagawa and Kamuikotan Belts, are reviewed on the basis of geochronological and geological data. The Higo metamorphic rocks and klippes of mid-Cretaceous metamorphic and granitic rocks exposed in the Sanbagawa Belt are considered to belong to the southern extension of the Abukuma Belt. The Ryoke-Higo-Abukuma Belt and Sanbagawa-Kamuikotan Belt are paired metamorphic belts which were formed simultaneously with the juxtaposition of Jurassic-Lower Cretaceous accretionary complexes of Northeast Japan and the Inner Zone of Southwest Japan. During the period of 130-95 Ma, Izanagi Plate subducted beneath eastern Asia. Its rapid plate motion caused a northward drift of Northeast Japan and the Outer Zone of Southwest Japan. The Sanbagawa-Kamuikotan Belt was formed in the eastern margin of the drifting landmass, while the Higo-Abukuma Belt was formed in its western portion. The Ryoke Belt was formed in the eastern part of the Inner Zone of Southwest Japan which was not displaced significantly. The mid-Cretaceous tectonics were completed at approximately 95 Ma when the subducting plate was changed from Izanagi Plate to Kula Plate. New accretion tectonics and subduction-related magmatism took place in the Shimanto Belt and the Inner Zone of Southwest Japan, respectively.View full abstractDownload PDF (1664K) -
Toshiaki Shimura, Toshiko Kawai, Shin-ichi Kagashima2002Volume 108Issue 5 Pages 347-350
Published: May 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSSpinel+quartz assemblage is one of the indicators of ultrahigh-temperature (UHT) metamorphism. Metamorphic xenoliths of granulite facies occur in the Sumikawa Granite, Niigata Prefecture. We found the spinel+ quartz assemblage from a pelitic metamorphic xenolith in the granite body. The metamorphic textures show a clockwise P-T-t path and the metamorphic reactions follow the order;Grt+Sil=Spl+Qtz, and Spl+Qtz=Crd. According to thermodynamic model, the spinel+quartz assemblage in this rock is thought to be stable at T>860°C and P>600MPa. This assemblage was presumably generated by metamorphism in the lower crust in the Uetsu area.View full abstractDownload PDF (757K) -
Takashi Sawaguchi, Ichiko Shimizu2002Volume 108Issue 5 Pages XI-XII
Published: 2002
Released on J-STAGE: December 14, 2010
JOURNAL FREE ACCESS
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