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Volume 106, Issue 8
Displaying 1-7 of 7 articles from this issue
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Tad James Choi, Yoshihiro Kubota2000 Volume 106 Issue 8 Pages 521-533
Published: August 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThis study clarified the stratigraphy of the Miocene series that occur in contact with pre-Neogene basement rocks in the Echigo-Yuzawa region. Based on new lithologic assemblages, the Miocene stratigraphy and the tectonic process of the Echigo-Yuzawa Basin were reconstructed. The Miocene formations are distributed in the Shiozawa and Tsuchitaru areas in the Echigo-Yuzawa region. The Shiozawa area comprises the newly proposed Shiozawa Formation (Maiko Breccia Member, Yoshiyama Shale Member and Komatsuzawa Basalt Member) and the Tsuchitaru area comprises the redefined Matsukawa Formation (newly proposed Tsuchitaru Sandstone and Basalt Member, Nakazato Tuff Member and Kandatsu Shale Member). A new record of planktonic foraminifer indicative of Blow's N.9 (Blow, 1969) was discovered from the Kandatsu Shale Member. The tectonic development of the Miocene Echigo-Yuzawa Basin is as follows : [Stage I] Basin generation due to collapse, forming a lacustrine lake (18-17 Ma).[Stage II] Subsidence accompanied with marine transgression and bimodal volcanism (17-15Ma).[Stage III] Continuous subsidence depositing mud (15 Ma).View full abstractDownload PDF (2190K) -
Akiko Omura2000 Volume 106 Issue 8 Pages 534-547
Published: August 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThe Nishikubiki area in the northwestern part of the Shin'etsu back-arc sedimentary basin, central Japan faces the Japan Sea. Turbidites and sandy siltstones of the Middle Miocene to Pleistocene are distributed in this area. The records of relative sea-level changes in the paleo-Japan Sea are preserved in the Middle Miocene to Pleistocene strata distributed in the sedimentary basin and it is known that the fourth-order depositional sequences in the sedimentary basin are controlled by eustasy. The purpose of this study is to investigate the driving force that formed the third-order depositional sequence in the Nishikubiki area.Facies analysis of the Pliocene strata in the Nishikubiki area has revealed that they were deposited in a series of environments ranging from basin plain to shelf. A third-order depositional sequence which was set up on the basis of stacking patterns of sedimentary facies is composed of a coarse gravelly channel-levee system on slope (lowstand systems tract), condensed section on outer shelf (transgressive systems tract) and shallowing upward facies succession on shelf (highstand systems tract).Maceral composition ratio and trace fossil assemblages are related to relative sea-level of the third-order depositional sequence. Vitrinite and sporinite of terrigenous origin have minimum content in the maximum flooding surface of the third-order depositional sequence. In contrast, they increase upward within the highstand systems tract of the sequence. Size and diversity of the trace fossils decrease upward in the transgressive systems tract of the third-order depositional sequence. On the other hand, they increase upward within the highstand systems tract of the sequence.The sea-level positions presumed from third-order depositional sequence, the assemblages of trace fossils, and maceral composition, coincide with the oxygen isotope curve. These results suggest that the third-order depositional sequence in this area was formed in response to global eustasy.View full abstractDownload PDF (3513K) -
Takeshi Yasuda, Hiromasa Kato, Kazue Tazaki2000 Volume 106 Issue 8 Pages 548-559
Published: August 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSJapan, acknowledged as a country with many volcanic activities, has various hot springs associated with colorful microbial mats (biomats). Biomineralization of calcite at Ogawa hot springs in Toyama Prefecture and Chugu hot springs in Ishikawa Prefecture have been investigated. Microbial calcite is formed in the both hot springs. Microbial calcite is covered with cyanobacteria to form thread-ball structure, so-called "Itodemari-structure". In the case of Ogawa hot springs, biomats appear with black and green stripes along the flow of hot spring water. Cyanobacteria are 2.7 μm wide and those living in the black biomats, contain abundant S and Ca with traces of Al, Si and P while those in the green biomats are 1.1 μm wide contain abundant S and Ca with traces of Si, P and Cl. Cyanobacteria living in the green biomats of Chugu hot springs are 0.9 μm wide and contained abundant Si, P, S, Cl, K, Ca and Fe.The color of the biomats is considered to reflect the size of the cyanobacteria. Under each biomat, calcite of different size was recognized. The mean diameter of calcite was 67.6 μm under the black and 38.0 μm under the green biomats, respectively. This phenomenon is also considered to be correlated with the size of the cyanobacteria. Light irradiated experiments showed that the fluctuation patterns of pH values are different between these biomats, due to the difference in the potential of photosynthesis. Further TEM observations showed that a number of spherical calcite are formed in bacterial adhesive, which suggests that bacteria in hot springs, play an important role for the nucleation of calcite under photosynthetic conditions, and control the crystal growth of calcite in forming thread-ball structure.View full abstractDownload PDF (3015K) -
Shuichi Tokuhashi, Tohru Danhara, Hideki Iwano2000 Volume 106 Issue 8 Pages 560-573
Published: August 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSThough a number of tuffs are intercalated in Amatsu, Kiyosumi and Anno Formation in ascending order, the upper part of the Neogene Awa Group, Boso Peninsula, central Japan, very few radiometric ages for these tuffs have been reported. The present authors report the fission track ages (FT ages) of zircon crystals from eight marker tuffs in those formations, i.e. Am19, Am40, Am61 Am78(Ok tuff), Am94, Ky21(Hk tuff), An49 and An 73 in ascending order.The followings are the main results of the present study.1) Dated FT ages for eight tuffs are as follows : Am 19 : 11.7±0.3Ma, Am 40 : 8.5±0.5Ma, Am 61 : 6.3±0.5Ma, Am 78 (Ok tuff) : 5.7±0.4Ma, Am 94 : 5.2±0.3Ma, Ky 21 (Hk tuff) : 4.5±0.2Ma, An 49 : 3.9±0.4Ma, An 73 : 3.7±0.2Ma. These data are the first FT ages following the proposal of IUGS Subcomission in 1990 for the tuffs in those formations.2) The present FT ages are a little younger than previously reported FT ages, while coincide well with the latest K-Ar ages for a few tuffs among eight tuffs dated in this paper.3) The present FT ages for tuffs in the lower to middle parts of the Amatsu Formation and in the Anno Formation coincide relatively well with biostratigraphic ages based on some key species of planktonic foraminifers and nannofossils, while those in the upper part of the Amatsu Formation and in the Kiyosumi Formation are significantly younger compared with those biostratigraphic ages. The cause of this difference is not known.View full abstractDownload PDF (2038K) -
K-Ar dating of hydrothermal alteration of the Yorii welded tuffs in the northeastern Kanto MountainsShiro Chikashige, Kazuo Kosaka2000 Volume 106 Issue 8 Pages 574-577
Published: August 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSYorii welded tuffs in the northeastern Kanto Mountains are penetratively crushed and strongly hydrothermally altered. They are younger constituents of the Atokura Nappe and their ages indicate the lower limit of the time when the Atokura Nappe thrusted up on the Sanbagawa belt.The whole rock K-Ar ages of the altered Yorii welded tuffs give 58.0±2.9 Ma which is consistent with zircon FT age (59.6 Ma). This suggests that the hydrothermal alteration followed the formation of welded tuffs at the last stage of cooling process. After this hydrothermal alteration, the Yorii welded tuffs thrusted up on the Sanbagawa belt.View full abstractDownload PDF (1331K) -
Rei Nakashima, Mahito Watanabe2000 Volume 106 Issue 8 Pages 578-581
Published: August 15, 2000
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSGeologic age of the Fortipecten takahashii (Yokoyama)-bearing lower part of the Horokaoshirarika Formation in the Ebishima area, Numata-cho, central Hokkaido, is assigned to the Neodenticula kamtschatica-Nilzschia rolandii Subzone (NPD7Ba) based on the diatom assemblage. The first occurrence age of F. takahashii in this area is estimated at 6.4-6.2 Ma and regarded as the oldest record of the species. On the other hand, the youngest record of F. takahashii is considered to be the occurrence from the early Pleistocene Yuchi Formation in northern Hokkaido. Thus, the geologic range of F. takahashii is estimated to be about five million years.View full abstractDownload PDF (835K) -
Tohru Yamanoi2000 Volume 106 Issue 8 Pages XV-XVI
Published: 2000
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESSDownload PDF (12713K)
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