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Volume 108, Issue 6
Displaying 1-6 of 6 articles from this issue
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Hiroyuki Hoshi, Hideki Iwano, Tohru Danhara2002 Volume 108 Issue 6 Pages 353-365
Published: June 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSFission-track (FT) dating was carried out on zircon crystals separated from various volcanic rocks of the Miocene Nijo Group, which is part of the Setouchi volcanic rocks. Twelve FT ages from a wide stratigraphic range were indistinguishable at the 2σ level and made a cluster around 15Ma. Accuracy of the FT ages was confirmed by semi- and confined-FT length measurements, as well as by repeated dating of the age standard (Fish Canyon Tuff). Our FT ages demonstrate that the Nijo Group was formed within a very short period around 15 Ma. It is concluded, based on the present results and reliable radio-metric ages from other areas, that the Setouchi volcanic activity occurred around 15 Ma. The well-known view that the intermediate to mafic rocks of the Setouchi volcanic rocks were formed around 12Ma while the felsic rocks around 14Ma is highly questionable. Additionally, it is confirmed that the FT method is useful in dating intermediate, glassy volcanic rocks if they contain zircon crystals regardless of origin. Furthermore, correlation of sporadically exposed Middle Miocene felsic pyroclastic flow deposits in the northern Kii Peninsula region is suggested from the similarity of their lithologies, FT ages, and the proportion of reddish zircon grains to colorless ones.View full abstractDownload PDF (2107K) -
Kazuto Kodama, Haruyoshi Maeda, Yasunari Shigeta, Tomoki Kase, Tohru T ...2002 Volume 108 Issue 6 Pages 366-384
Published: June 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Cretaceous clastic sequences in the Naiba area of South Sakhalin, Russian Far East, have been investigated in terms of macrofossil stratigraphy and magnetostratigraphy in conjunction with detailed field mapping since 1990. This paper provides an overview of the lithostratigrahy and magneto-biostratigraphy of the Upper Cretaceous portions that are well-exposed along the River Naiba and its tributaries. The Cretaceous strata in Naiba are subdivided lithologically into the Ai, Naiba, Bykov, and Krasnoyarka Formations, in which the Krasnoyarka Formation and the upper part of the Bykov Formation yield abundant ammonoids and inoceramids that commonly occur in the "Upper Yezo Group" and the "Hakobuchi Group" in central Hokkaido. A stratigraphic sequence of magnetic polarity reversals consisting of 15 magnetozones can be established from the upper Bykov and the entire Krasnoyarka Formations. Although cosmopolitan marker species are few, a continuous biostratigraphic section ranging from Jimboiceras mihoense Zone (Coniacian) to Pachydiscus subcompressus Zone (Maastrichtian) via zones of Canadoceras spp. (Campanian), all of which characterize the North Pacific Realm, is typically observable in the Naiba area. The biostratigraphic age assignments using these ammonoids and other molluscs lead to the correlation of the geomagnetic polarity sequence with polarity chrons from C34n, the Cretaceous long normal interval, through C30n in the upper Maastrichtian.View full abstractDownload PDF (2251K) -
Kazuo Kosaka, Shiro Chikashige2002 Volume 108 Issue 6 Pages 385-393
Published: June 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSA fault zone juxtaposes the allochthonous Yorii Formation and Yorii Welded Tuff overlying the Mikabu greenstones of the Sambagawa belt. This boundary fault zone, about 5 m in width, is composed of breccia and 3∼5 cm-wide gouge zone along the fault surface trending N51°E, which is different from the fault systems of this area striking WNW-ESE or N-S. The hanging wall rock, Yorii Formation, exhibits fault rock fabrics such as imbrication structures and composite planar fabrics, suggesting a dextral sense of shear. Restored strike of the boundary fault considered from the rotation of the Kanto Mountains accompanying the formation of the Japan Sea (clockwise rotation of 94°) and that in the nappe-forming process of the Yorii Formation (counterclochwise rotation of 73°) indicates about N30°E, suggesting the parallelism to the Median Tectnic Line (MTL) of the Southwest Japan before the opening of the Japan Sea. Thus, the fault zone is considered to have occurred accomanied with the activity of the MTL. The K-Ar ages were determined on fine fractions of clay minerals (<2μm) extracted from the fault gouge, resulting 23.6 ±0.6 Ma. This suggests the last motion of the fault having occurred before this age and is compared to the Akaishi phase (15-27 Ma) in the movement phase of the MTL. As a result, the boundary fault zone is considered to have formed in association with the activity of the MTL at the heimat of the Yorii Formation and the Yorii Welded Tuff in the Inner Zone of the Southwest Japan, stayed there until 23.6 Ma and thereafter was brought to the present place together with the Yorii Formation and the Yorii Welded Tuff.View full abstractDownload PDF (1324K) -
Ritsuo Nomura, Koji Seto2002 Volume 108 Issue 6 Pages 394-408_1
Published: June 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSForaminiferal events induced by man-made impacts are modern analogues that are significant in interpreting pre-historic environmental changes. The Honjo area of the brackish Lake Nakaumi of Japan is an ideal place to examine the process of foraminiferal disappearance. The Honjo area is mostly enclosed and has a maximum depth of 5.5m. Its environment has been changed artificially by a land reclamation project, with the construction of dikes, a barrier gate, and sediment dredging. Three steps of assemblage change are recognized by the time of foraminiferal disappearance based on the thanatocoenoses. In the first step, the original marine assemblage was destroyed abruptly, over a time interval of less than 3 years. A typical brackish assemblage appeared in the second step, and continued to develop over the next 7-10 years. Foraminiferal assemblages disappeared in the third step. These three steps correspond to salinity changes of the Honjo waters. The first step was caused by the cessation of marine water upstream, and the second reflects more restricted circulation of brackish water, with higher salinity bottom water and less saline surface water. The third and final step was caused by collapse of the halocline, which induced thorough mixing of the bottom and surface water, and wind-stressed winnowing of bottom sediments. This resulted in the oxidation of deposited organic and sulfide materials. The final step is more significant for the survival of brackish foraminifera. Foraminiferal disappearance in the Honjo area was completed in maximum 10 years.View full abstractDownload PDF (2057K) -
Ken-ichi Nishiyama, Yukinori Matsukura2002 Volume 108 Issue 6 Pages 410-413
Published: June 15, 2002
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSSequential changes in mineralogical properties due to weathering during 350 ka were examined using sandstone gravel in fluvial terrace deposits with a known emergence time which were distributed in the Miyazaki Plain, South Kyushu. Fe is increasingly concentrated and Si and alkali elements such as K, Na are increasingly leached from the sandstone matrix with the progress of weathering. Pores of gravel increase with weathering and can be correlated to the decrease of Si, K and Na contents.View full abstractDownload PDF (675K) -
Masayuki Sakakibara, Yoshio Inouchi, Masakazu Nara2002 Volume 108 Issue 6 Pages XIII-XIV
Published: 2002
Released on J-STAGE: December 14, 2010
JOURNAL FREE ACCESS
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