Earth Science (Chikyu Kagaku)

Discovery of crab fossils from the bottom of Tosa Bay off Shikoku, Japan(Photo)

Tatsuya Sakumoto

Article type: Article
2000 Volume 54 Issue 6 Pages 355-356
Published: November 25, 2000
Released on J-STAGE: July 14, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.54.6_355

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The modes of occurrence of carbonatite and glimmerite in the Shandong province, China(Photo)

Toshiro Morikiyo, Akiko Goto, Toshiaki Takahashi

Article type: Article
2000 Volume 54 Issue 6 Pages 357-359
Published: November 25, 2000
Released on J-STAGE: July 14, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.54.6_357

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Disaster of the western Tottori earthquake in 2000(Photo)

Earthquake Disaster Research Group of Shimane University

Article type: Article
2000 Volume 54 Issue 6 Pages 360-362
Published: November 25, 2000
Released on J-STAGE: July 14, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.54.6_360

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An Eocene cold-water molluscan assemblage from the Poronai Formation in the Mikasa area, central Hokkaido, northern Japan

Akihiko Suzuki

Article type: Article
2000 Volume 54 Issue 6 Pages 363-368
Published: November 25, 2000
Released on J-STAGE: July 14, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.54.6_363

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Molluscan fossils from the lower part of the Eocene Poronai Formation (upper middle Eocene) of the Mikasa area of central Hokkaido, northern Japan comprise ten species of bivalves and two species of gastropods. The Malletia poronaica-Cyclocardia tokudai assemblage contains many cold-water genera, such as Malletia, Acila, Portlandia, Cyclocardia, and Neptunea. This assemblage is dominated by M. poronaica and C. tokudai and is associated with P. watasei, Conchocele bisecta, and Periploma besshoense. The Malletia-Cyclocardia assemblage indicates lower sublittoral to upper bathyal, muddy bottom environment.

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Arsenic distribution in the surface geology of the Niigata Plain, central Japan ―Source supply of arsenic in arsenic contaminated ground water problem, Part 1―

Yoshihiro Kubota, Yutaka Ishiyama, Daiki Yokota

Article type: Article
2000 Volume 54 Issue 6 Pages 369-379
Published: November 25, 2000
Released on J-STAGE: July 14, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.54.6_369

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In order to consider source supply of arsenic in arsenic contaminated ground waters, the arsenic distribution in the surface geology of the Niigata Plain was investigated from the view point of alluvium forming processes. The following conclusions were reached: 1) Arsenic concentration in the surface geology is 0.1〜2ppm for igneous rocks, 3〜5ppm for sandstones and 8〜12ppm for mudstones. Arsenic concentration in muddy facies were higher than others. Arsenic concentration increases in the soil horizons of the soil layer compared with the weathered and parent material layer. 2) Arsenic concentration of natural levee/back swamp sediments is 2〜14ppm which is comparable to the strata in the hinterland. Arsenic concentration of lacustrine organic clay is 25〜64ppm, which is extremely high. 3) Arsenic concentration in the surface geology that are composed of Tertiary, Pleistocene, and Holocene is related to the ignition loss and clay particles content respectively. The increase in arsenic and iron concentrations in river waters is associated with the ascent of turbidity. The comparative arsenic concentrations in alluvium sediments and the strata in the hinterland could be considered that the supply of arsenic into the alluvium plain occurs as an ordinary geologic phenomena during the alluvium forming process. The organic muddy layer which contains high arsenic will be an important target in solving arsenic concentration and the exploration to safe underground waters.

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Granitic masses around Lake Biwa, southwest Japan : Part 5. The Tanakami Granite pluton

Collaborative Research Group for the Granites around Lake Biwa

Article type: Article
2000 Volume 54 Issue 6 Pages 380-392
Published: November 25, 2000
Released on J-STAGE: July 14, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.54.6_380

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The Tanakami Granite pluton, 20km(E-W)×8km(N-S) in extent, is the southernmost constituent of the ring-bodies associated with the formation of the Biwa-ko Cauldron of Late Cretaceous age. It was emplaced within the Jurassic accretional complex of the Tamba Belt, intruding and enclosing the Kannonji Granodiorite to the north. The rocks are all biotite granites, and classified into the following four rock-types according to the grain size and texture: medium to coarse-grained biotiote granite, medium to coarse-grained porphyritic biotite granite, fine to medium-grained porphyritic biotiote granite and fine-grained biotite granite. In the northeastern part of the pluton, there are zonal arrangements of granite porphyry dykes, cutting the granite. These dykes represent the ring-dykes associated with the Biwa-ko Cauldron. Within the Kannonji Granodiorite and its neighbourhood, occurs quartz porphyry bearing the xenoliths of felsic pyroclastic rocks, probably a member of the Koto Rhyolites. There are geochemical differences between the Tanakami Granite and the Kannonji Granodiorite. The dyke rocks are geochemically different from the Tanakami Granite, too. The Tanakami Granite is petrogenetically consanguineous with the granites around Lake Biwa.

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Reconstruction of the delta and estuary systems and sequence stratigraphy of the Plio-Peistocene strata in the Higashikubiki Hills, Niigata Prefecture, central Japan

Koichi Hoyanagi, Daisuke Ikedzu, Yuka Shimizu, Akiko Omura

Article type: Article
2000 Volume 54 Issue 6 Pages 393-404
Published: November 25, 2000
Released on J-STAGE: July 14, 2017

DOIhttps://doi.org/10.15080/agcjchikyukagaku.54.6_393

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The study area is located in the northern part of Higashikubiki Hills in the Niigata Prefecture, central Japan. The studied area is underlain by the Plio-Pleistocene Uonuma Group and Haizume Formation, covers the Yamaya Anticline. The Haizume Formation, which is equivalent to the Middle Uonuma Formation, distributed only on the western wing. The Uonuma Group is represented by two types of depositional cycles. One shows shallowing upward successions deposited on shelf to shoreface environments of wave-dominated delta system and is accompanied with transgressive lag at the base. The other shows repetition of fluvial-estuary sediments. Depositional systems of the Uonuma Group changed from delta to estuary in the early Pleistocene. The Haizume Formation consists of shallowing upward successions, which resulted from deposition in margin of the wave-dominated delta system. Correlation of the ash layers indicates deposition of the Haizume Formation occurred as a continuation from the delta system when the systems changed in the Uonuma Group. The sequence stratigraphic analysis has revealed seven depositional sequences in the Uonuma Group and the Haizume Formation. Each sequence has a duration of 0.14My on average. These fourth-order depositional sequences are correlative with those of the other areas in Niigata Sedimentary Basin. The fourth-order sequence boundaries were formed concurrently with the periods of fourth-order sea-level fall in the oxygen isotope curve.

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