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Volume 108, Issue 2
Displaying 1-7 of 7 articles from this issue
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Masakazu Niwa, Kazuhiro Tsukada, Satoru Kojima2002 Volume 108 Issue 2 Pages 75-87
Published: February 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSA reassessment of the Junigatake Formation, composed mainly of mudstone, sandstone, and felsic tuff, in Nyukawa Village, Gifu Prefecture suggests it is Permian in age. The formation is generally considered to be a Jurassic melange that includes Permian exotic blocks. The Junigatake Formation can be divided into Lower and Upper members. The Lower Member consists of broken beds of sandstone associated with mudstone, while the Upper Member consists of alternating beds of sandstone and mudstone interbedded in the middle part with alternating beds of felsic tuff and mudstone. The felsic tuff yields abundant Late Permian radiolarians (e.g. Follicucullus charveti). The Junigatake Formation can be correlated with the Oi Formation of the Ultra-Tamba Belt on the basis of lithologic, stratigraphic, and radiolarian similarities. This is the first report of a Permian formation from the Hida Mountains correlatable with part of the Ultra-Tamba Belt.View full abstractDownload PDF (3532K) -
Teruki Oikawa, Mitsuru Okuno, Toshio Nakamura2002 Volume 108 Issue 2 Pages 88-102
Published: February 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe past 3000 years' eruptive history of the Yakedake volcano, located in the southern part of the Northern Japan Alps, central Japan, has been revealed by tephrostratigraphy as well as radiocarbon dating. We identified eleven tephra layers from the Yakedake volcano. These are named Yakedake-Tougezawa tephra group (Ykd-T) and are stratigraphically divided into two sub-groups ; the lower (Ykd-Tl) and the upper (Ykd-Tu) subgroups. The tephra layers are very poorly sorted and mainly composed of altered fine-grained ash including lithic fragments. Only one tephra bed, Ykd-TNkb, is a fine-grained vitric ash. Based on stratigraphy, the Ykd-TNkb could have been supplied by the eruption of the Nakao pyroclastic flows and the Yakedake lava dome (ca. 2300 cal BP). Judging from observations on the AD 1962 historical eruptive products, there is little chance to detect a phreatic eruption smaller than 1×106 m3 eruption volume in geologic records. Frequency of phreatic eruptions greater than 1×106 m3 during the past 3000 years is estimated to be 2-8 times/ky. On the other hand, magmatic eruptions occurred only once during a few thousand years. This revealed that Yakedake volcano was the most active volcano in the Norikura volcanic zone during the past 10, 000 years. On the basis of calibrated radiocarbon data, stratigraphic relation and historical records, it is probable that Ykd-Tu2 tephra erupted in AD685, Ykd-Tu7 tephra in AD1746 and Ykd-Tu8 tephra in AD 1907-1939.View full abstractDownload PDF (2185K) -
Yu'suke Kubo, Wonn Soh, Hideaki Machiyama, Hidekazu Tokuyama2002 Volume 108 Issue 2 Pages 103-113
Published: February 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSDuring the high-resolution side-scan sonar survey off Kozu Sima, Nii-jima and Miyake Sima Islands, a northern part of the Izu-Mariana Ridge, various kinds of bedforms such as 2D dunes, 3D dunes and sand ribbons, were observed on the sea floor at a depth of 100-400 m. These bedforms display sharp profiles, which are interpreted as fresh bedforms produced and maintained by the Kuroshio Current. The distribution of these bedforms indicates that the Kuroshio Current passes a narrow channel between southern coast of Kozu Sima and northern end of Kozu-nakase to enter the study area, and that the flows over Kozu-nakase is too weak to produce large bedforms on the sea floor. The flow passing the narrow channel extends to develop very large dunes on the sea floor at a depth of 350 m, which have the maximum wavelength of c. 500 m and height of c. 15 m.The flow velocities estimated from the necessary conditions for dune formation and the observed dune morphology show that i) the surface flow velocity required to develop dunes on a initially flat bed is 2-3 m/s, and ii) that required to maintain the observed very large dunes is 1 m/s. These values are fairly consistent with the observed flow velocity in this area.View full abstractDownload PDF (2359K) -
Koji Hamano, Keiji Iwata, Makoto Kawamura, /, /, /, [in Japanese], ...2002 Volume 108 Issue 2 Pages 114-122
Published: February 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSDevonian conodonts were newly found from red chert of the Hayachine Belt in southeast Morioka district, Northeast Japan. Hayachine Belt is an accretionary complex composed of deformed clastic rocks and greenstones, also constituting a boundary zone between the North and South Kitakami Belts. The conodont assemblage is characterized by the presence of Palmatolepis indicating Late Devonian age (early Famennian). This is the first report of Devonian oceanic rocks in Japanese Islands, confirmed by fossil occurrence.The conodont-bearing chert occurs with bedded iron ore deposits as a lenticular body within greenstone. Chemical analyses for the greenstone show two rock types, within-plate alkali basalts and MORB to within-plate tholeiites. Tholeiites occurring with iron ore deposits indicate the compositional range of N-MORB to P-MORB. The greenstone of the Hayachine Belt is thus considered to have been formed by ocean-floor and oceanic island volcanisms with hydrothermal activity in Late Devonian time.View full abstractDownload PDF (1515K) -
Hajime Sakai, Katsuki Kurokawa2002 Volume 108 Issue 2 Pages 123-126
Published: February 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Tsuike volcanic ash bed (TsA)(newly defined) is found in the marine silt-sand beds of the Uonuma Group over 24 km in the Uonuma Hill. The Tsuike volcanic ash bed is 6 to 35 cm thick, fine sand to silt sized, mainly composed of vesiculated thin glass shards with a few crystals of plagioclase, hornblende and biotite. It is intercalated about 25 m above the Tsujimatagawa volcanic ash bed (1.75 Ma), and about 70 m below the SK110 volcanic ash bed (1.61 Ma) at Tagawa, Tokamachi city and the age of the Tsuike volcanic ash bed was estimated to be about 1.7 Ma. The ash bed is also identified in the Niitsu Hill at Jojo, Kamo City (4 cm thick) and Sugenosawa, Gosen City (5 cm thick). At Sugenosawa in central Niitsu Hill, it is intercalated about 5 m above the basal part of the marine mud of the Taira Formation. The Tsuike volcanic ash bed is thus useful marker tephra for correlation of the 1.7 Ma marine beds widely interbedded in the Uonuma Group and Taira Formation in Niigata Prefecture.View full abstractDownload PDF (1063K) -
Masatoshi Shiba, Teruo Mikawa, Rikako Shiba(Sato), Kazuhiko Takahashi2002 Volume 108 Issue 2 Pages 127-130
Published: February 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSCharacteristics of sandstones from so-called "Hidaka Supergroup" in the Erimo area were discussed based on modal compositions and chemical compositions of heavy minerals by means of an electron-probe microanalyser. The modal compositions suggest transitional to undissected arc provenance. Chemical features of detrital grains indicate that intermediate to basic rocks, partly metamorphosed to intermediate grade, and peridotites of arc or fore-arc origin were exposed on the land. These formations in the Erimo area can be correlated to the Nakanogawa Group in the Hidaka Belt.View full abstractDownload PDF (660K) -
Michio Morino, Tsugio Shiota, Ikuo Hara, Saori Ikeda, Yoshihiro Tanaka ...2002 Volume 108 Issue 2 Pages III-IV
Published: 2002
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESS
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