Earth Science (Chikyu Kagaku)
Online ISSN : 2189-7212
Print ISSN : 0366-6611
Volume 73 , Issue 2
Showing 1-6 articles out of 6 articles from the selected issue
Outcrops of Japan
Research Article
  • Yoshihiro UJIIÉ, Kurumi HARADA, Natsuki HAMADA, Naoki HASEGAWA, Maiko ...
    Type: research-article
    2019 Volume 73 Issue 2 Pages 61-74
    Published: April 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    Vitrinite reflectance (Ro) is one of the most confidential parameters for organic maturation. Although in case of concentrated organic matter such as coal it is easy to measure reflectance of vitrinite macerals, in case of dispersed organic materials in argillaceous sediments, it is not easy because those organic grains are often smaller than a spot of measurement under a microscope. We have measured both reflectance and colour (RGB intensities) of the same grains of vitrinite chemically isolated from 37 muddy rock samples of the Miocene from three boreholes in northern Japan. The reflectance of vitrinite grains are measured with the microscope of Axio Imager A2m connected to the microphotometer of MSP200 made by Carl Zeiss, and then images of the macerals under the reflecting microscope are taken with a digital camera. The colour of image of individual macerals are analyzed in a computer with image processing software. The condition on taking photos under the reflecting microscope is following: the light reflected from the standard of Gadolinium-Gallium-Garnet 11-87 (reflectance 1.719% in immersion oil) is set 20% of the maximum intensity of lighting of the microscope at colour temperature of 3200 K and exposure time of a camera is 31.0 ms.

    There is very strong correlation between Ro and each RGB intensity of vitrinite grains, whose correlation coefficients are all over 0.95. Three approximate expressions are gained with those data.

    Ro = 0.0000788R² + 0.000588R - 0.046 from R intensity of colour,

    Ro = 0.00010G² - 0.0086G + 0.410 from G intensity of colour,

    Ro = 0.00020B² - 0.0213B + 0.920 from B intensity of colour

    Values of Ro estimated from each RGB intensity of vitrinite grains are close to those of Ro measured, and are plotted on or near an increasing line of measured Ro on a diagram of Ro vs depth of each three boreholes. Therefore, it is concluded that the organic maturity of sedimentary rocks which include only too small vitrinite grains to measure reflectance can be estimated from their colour.

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  • Implications for provenance, weathering and sorting of sediments
    Kana TAKASU, Barry ROSER, Atsushi KAMEI
    Type: research-article
    2019 Volume 73 Issue 2 Pages 75-92
    Published: April 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    Lake Biwa, the largest lake in Japan, first formed some four million years ago. The main lithotypes in the catchment of Lake Biwa are Paleozoic-Mesozoic Tamba terrane rocks, late Cretaceous to Paleogene granites and associated rhyolites, and the Plio-Pleistocene Kobiwako Group sediments and younger alluvium. Thirty-three river sediments from the mouths of 17 rivers flowing into Lake Biwa were analyzed by X-ray fluorescence to determine their bulk geochemical compositions. These were then compared with published data for the main source rocks. Samples were divided into three categories according to their main sources, i.e. Group 1: alluvium-derived, Group 2: granitoid-derived and Group 3: Tamba terrane-derived. Group 2 sediments are marked by depletion in MgO, CaO, Na2O, Cr, Ni, Sr, V and enrichment in Rb relative to average upper continental crust (UCC). Group 3 sediments show lesser depletion in CaO, Na2O and Sr, but are richer in ferromagnesian elements, with contents closer to UCC. Group 1 samples, the source sediments of which were themselves derived from Groups 2 and 3 sources, show intermediate and more diverse compositions.

    Established geochemical diagrams show that while the sediments generally retain the geochemical fingerprints of their source rocks, significant shifts in composition have occurred due to source weathering and recycling. Sorting and heavy mineral accumulation have less effect, although some Group 1 samples show zircon concentration, probably due to reworking of alluvium which was mainly derived from granitoids. Group 3 samples also show V enrichment, possibly due to presence of fine-grained magnetite and clays. The present study suggests that the chemical composition of river mouth sediments is useful not only for its provenance research but also interpretation of the weathering and sorting.

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Special issue: The culture geology of Hokkaido(2)
Review
  • obsidian/siliceous shale, serpentinite related earthenware and precious stone
    Takayuki KATOH, Tomoaki IIDA, Mayuko YONEJIMA, Koki SAITO
    Type: review
    2019 Volume 73 Issue 2 Pages 93-102
    Published: April 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    The stone age people of Hokkaido obtained siliceous shale and obsidian for flake tools from various localities. At the archaeological site of Washinoki-4, the source of siliceous shale was at a short distance of 16 km from the remains, and this material occupied 66% of the flake tools. Conversely, the source of obsidian was 100 km further from the remains, and it occupied 25% of the tools. The “mental value” of siliceous shale is low and that of obsidian is high, because a high mental value for siliceous shale would require over 80% occupation at Washinoki-4. The raw material for serpentinite-related earthenware was possibly carried from landslide deposits of the Mitsuishi serpentinite Mélange. At the archaeological site of Ocharasenai, the raw material was probably sourced from the Mitsuishi serpentinite mélange at a distance of 90 km. The fact that serpentinite-related earthenware was used despite its brittleness compared with the strong common earthenware suggests that the serpentinite-related earthenware had a somewhat high mental value. Chrome spinel inclusions in chlorite and talc precious stones indicate that they were derived from the serpentinite. These stones can be inferred to have high mental value, because the distance to the source for chlorite from the Nishishimamatsu-5 archaeological site is 65 km, and that of talc from the Tatesaki archaeological site is 20 km.

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Research Article
  • Akihiko SUZUKI, Takafumi ENYA
    Type: research-article
    2019 Volume 73 Issue 2 Pages 103-110
    Published: April 25, 2019
    Released: June 04, 2020
    JOURNALS FREE ACCESS

    Various oddly-shaped rocks on the coast of Okushiri Island in southwestern Hokkaido are examined based upon field geology and folklore literature. Characteristic twelve geosites of the island are described with their geology and chronostratigraphy, and determined geological attributions of oddly-shaped rocks. Based on chronostratigraphy, oddly-shaped rocks of Okushiri Island are divided into three horizons, i.e. Cretaceous, middle Miocene and lower Pliocene, in ascending order.

    The Cretaceous horizon is characterized by Mottate Iwa Rock (granodiorite) after block weathering and wave erosion. The Miocene horizon is represented by Cape Inaho and Benten, which are made of intrusions of volcanic rocks in the Tsurikake Formation after differential weathering and erosion. The Pliocene horizon is characterized by Nabetsuru Iwa, Hoya Ishi and Kabuto Iwa Rocks, which are composed of intrusions of volcanic rocks in the Hotokezawa and Yoneoka Formations after differential weathering and erosion. Folklore of oddly-shaped rocks in Okushiri Island are mainly local origin, but some folklore is related to the legend of Yoshitsune.

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