The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 114, Issue 9
Displaying 1-4 of 4 articles from this issue
Articles
  • Toshio Kohno, Satoshi Nakano, Norimasa Shimobayashi
    2008 Volume 114 Issue 9 Pages 435-446
    Published: September 15, 2008
    Released on J-STAGE: May 30, 2009
    JOURNAL FREE ACCESS
    Small alkali feldspars with mantle-zoning occur in miaroritic pegmatites in the fine-grained biotite granite as a roof facies of the Tanakami Granite pluton, Otsu, Shiga, southwest Japan. The mantle- zoning visible to the naked eye consists of white alkali feldspar in the core and non-colored, transparent alkali feldspar in the thin rim. The core alkali feldspar is lamellae-patch microperthite, but the rim alkali feldspar is microscopically featureless. The average composition of the core alkari feldspar is Or76Ab24. The compositions of the rim are comparable to the extreme compositions of the host Or-rich phase. The formation processes of the alkali feldspar are estimated as follows. The alkali feldspar in the core was crystallized at the temperature about 550∼600°C (PH2O=2−3 kb), then was transformed to cryptoperthite at nearly 450°C. The cryptoperthite was coarsened to microperthite, and, eventually, the alkali feldspar in the rim was crystallized at nearly 200°C together with the microperthite in the core, which underwent coarsening and albitization. The last very-low temperatures reactions should provide a new insight to the understanding of the environments and cooling conditions of pegmatite.
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  • Yu Horiuchi, Ken-ichiro Hisada, Yong Il Lee
    2008 Volume 114 Issue 9 Pages 447-460
    Published: September 15, 2008
    Released on J-STAGE: May 30, 2009
    JOURNAL FREE ACCESS
    This paper described the depositional facies and systems of the late Early Cretaceous Shiohama Formation, the lowest part of the Shimonoseki Subgroup, exposed at Ajironohana headland, west of Yoshimi, Yamaguchi Prefecture. A special attention was also paid to the occurrence of red beds and pedogenic features in this formation.
    The Shiohama Formation can be divided lithologically into the Lower, Middle and Upper members, in which very fine-grained sandstone and conglomerate, conglomerate, and volcanogenic conglomeratic sandstone are predominated, respectively. The Lower member consists largely of interbeds of sediment gravity-flow and thick floodplain deposits. Sediment gravity-flow deposits are components of debris-flow dominated alluvial fan, whereas floodplain deposits with abundant paleosols are comparable to the distal part of the sheet-flooding dominated alluvial fan or the overbank fines of alluvial plain river probably. The Middle member is composed mostly of thick piles of sediment gravity-flow deposits accumulated on the alluvial fan as debris-flow lobes. The Upper member is made up of sandy bedforms with small amount of floodplain deposits, in which some volcaniclastic sediments are intercalated as debris-flow lobes and lag deposits. The paleosols in this member are less-developed, and are often covered by sheeted sandy sediments by plane-bed flow. It is inferred, therefore, that the Upper member was deposited on the middle part of the sheet-flooding dominated alluvial fan.
    Some red beds in the Shiohama Formation could be identified to the paleosol on the basis of soil feature such as trace of life, soil horizons and soil structures. They are developed mainly in the Lower and Upper members containing calcrete abundantly.
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  • Yoko Michiguchi
    2008 Volume 114 Issue 9 Pages 461-473
    Published: September 15, 2008
    Released on J-STAGE: May 30, 2009
    JOURNAL FREE ACCESS
    Chaotic blocks occur within a coherent zone of the Nishizaki Formation at Nishikawana to the southwest of Tateyama in southern Boso Peninsula. Based on detailed field observations and related laboratory studies the following facts were clarified, 1) the chaotic blocks are divided into 6 domains, 2) these domains are highly folded and faulted and generally overturned, 3) liquefied pumiceous sediments are found along fault zones between the adjacent coherent zone and the chaotic blocks, 4) the equivalent strata of the chaotic blocks are not found or correlated in the neighboring areas. The observations suggest that the chaotic blocks were emplaced by submarine landslide which was caused by liquefaction, and all the domains were subsequently stacked by thrust faults at the toe of the slide body. The domains were slid from S to N as shown by the fold axes analysis. This sliding direction is mostly perpendicular to the proto-Sagami Trough, suggesting that the submarine landslide have occurred on a forearc slope of the Izu Arc.
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  • Ken-ichi Takahashi, Yasuo Kondo, Nobuhiro Kotake
    2008 Volume 114 Issue 9 Pages 474-492
    Published: September 15, 2008
    Released on J-STAGE: May 30, 2009
    JOURNAL FREE ACCESS
    Small-scale transgressive-regressive cycles can be recognized in the lower part of the Tatamigaura Sandstone Member of the early middle Miocene Tougane Formation, Shimane, Japan. Each cycle ranges from 0.9 m to 6 m in thickness. Facies analysis shows that sedimentary environments ranged from shoreface to inner shelf. Transgressive deposits generally consist of shell bed or shell rich-horizon on or above an erosional surface. These shell beds were formed under conditions of shoreface erosion and sediment bypassing in the early transgressive phase. Highstand deposits are poorly fossiliferous and are characterized by physical sedimentary structures such as hummocky cross-stratification. Each cycle probably resulted by high frequency glacio-eustastatic sea-level changes inferred to be less than 50 m.
    Eight macroinvertebrate fossil associations or assemblages are recognized in the cycles. Distribution pattern of macroinvertebrate fossils is interpreted to have been controlled by changing sedimentary regime and sedimentation rate. Shoreface erosion and sediment bypassing in early transgressive phase may have prevented colonization by most of the shelly benthos, except for Glycymeris which are adapted to environments where erosion is common. High diversity associations including Anadara, Dosinia and Clementia appeared during the maximum flooding phase, in physically stable environments characterized by low rate of sedimentation due to reduced sediment supply, where risk of being buried alive or being reworked is low. Physical disturbance was also common and rapid deposition occurred in highstand/regressive phase. Only bivalves adapted to this habitat are found here, such as those possessing long siphons (Panopea) or swimming ability (Mizuhopecten).
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