The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 110, Issue 6
Displaying 1-6 of 6 articles from this issue
Articles
  • Masami Otake
    2004 Volume 110 Issue 6 Pages 331-347
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Two voluminous volcaniclastic sediment gravity flow deposits (PT-1 and PT-2) are widely distributed in the Pleistocene Kannodai Formation, which accumulated in a confined caldera lake of the Akakura caldera, Northeast Japan. The sediments of PT-1 and PT-2 show successive fining-upward sequence divided into five facies; A, B, C, D and E, in ascending order. The entire sequence begins with Facies A which consists of a large amount of pumice and lithic clasts displaying non-stratified and poorly-sorted. Facies B consists of interbedded pumice- and lithic-dominated beds showing parallel stratifications, where ungrading and partly inverse grading of pumice clasts occur. Grain size of the clasts and thickness of each bed gradually decrease upward into overlying Facies C. Facies C shows mostly parallel-stratifications and inverse grading with outsized pumice blocks in places. Facies D consists of bedded coarse ash and commonly contains load casts. Facies E consists of massive, well-sorted fine ash and contains dish structures.
    The lithofacies and sedimentary processes reveal that the overall sequences of PT-1 and PT-2 emplaced wholly subaqueous environment without evidence for hot, gas-supported state of emplacement. PT-1 and PT-2 are roughly similar to the sequences deposited by single high- and low-density turbidity current. However, there are remarkable features: (1) Facies B lack the deposits from traction carpets produced by dispersive pressure at the base of a high-density flow; and (2) the presence of the dish structure in Facies E, indicating the rapid sedimentation and fluid escaping after fall-out sedimentation of fine ash. These features suggest that (1) the volcaniclastic materials were presumably deposited from sediment gravity flows generated by progressive sorting within subaqueous eruption column, and (2) the confined basin (caldera lake) inhibited the dispersion of the volcaniclastic materials, not like an open ocean in which the sediments would extend freely toward the basin. The sediments of PT-1 and PT-2 are interpreted as the product of the post-caldera volcanic activity of the Mimizuku-yama dacite located in the southeastern part of the caldera.
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  • Hidetoshi Hara, Kenji Kashiwagi
    2004 Volume 110 Issue 6 Pages 348-362
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    The Jurassic accretionary complex of the Ashio Terrane, distributed in the Kuromatagawa region of Niigata Prefecture, central Japan, is divided into two tectonostratigraphic units; the Kuromatagawa and the Oshirakawa Complexes, based on lithology and geologic age.
    The Kuromatagawa Complex is composed mainly of coherent sequences of interbedded sandstone and shale, and slabs of basaltic rocks and chert ranging from 300 to 500 m and over in thickness. The Oshirakawa Complex consists of melange-type rocks and slabs of basaltic rocks, limestone, chert and sandstone ranging from 50 to 300 m in thickness. It is characterized by melange-type rocks including basaltic rocks and sandstone less than 50 m in thickness within pelitic matrix. According to radiolarian fossils obtained from shale, the accretionary age of the Kuromatagawa Complex ranges from middle Middle to early Late Jurassic (late Bathonian - Oxfordian), and the Oshirakawa Complex was accreted in middle Early Jurassic (late Sinemurian - early Pliensbachian).
    Considering the lithology and geologic age, the Kuromatagawa and the Oshirakawa Complexes are correlated with tectonostratigraphic units of the Tamba-Mino Terrane in the Inner Zone of Southwest Japan. The Oshirakawa Complex is one of the early Jurassic complexes which is the oldest in the Tamba-Mino-Ashio Terrane, and occupies the structurally uppermost unit in the Tamba-Mino-Ashio accretionary wedge.
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  • Naotatsu Shikazono, Itsuki Muto
    2004 Volume 110 Issue 6 Pages 363-371
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    Mizunami Group of Miocene sedimentary rocks in the Tono sand-stone-type uranium mine area, central Japan is composed of lacustrine lignite-bearing Toki Formation (20∼18 Ma) and marine Akeyo (16∼15 Ma) and Oidawara (5∼0.7 Ma) tuffaceous mudstone and siltstone Formations.
    Pyrite in the Toki Formation is abundant, and euhedral-subhedral in form and has anomalously high δ34S values ranging from +4 to +57‰, whereas pyrite in the Akeyo is small in amounts, and framboidal in form and has low δ34S values (−28∼−8‰). The sulfur isotopic compositions of pyrite, and mineralogical and geological features of the Tono area are similar to the lake sediments of Morrison Formation of Western U.S.A. in which large sandstone-type uranium deposits occur. These similarities suggest that sedimentary and diagenetic processes occurred under alkaline conditions in middle Miocene age (18∼16 Ma) in the Mizunami district. Unusually high δ34S value of euhedral-subhedral pyrite in the Toki Formation is thought to have been caused by the extensive reduction of SO42- with high δ34S value. This high δ34S of SO42- was caused by the increase in δ34S value of seawater due to the mixing of seawater with fresh water accompanied by the precipitation of framboidal pyrite with low δ34S. The alkaline conditions are considered to have been caused by the change from fresh water environment to marine environment and from low temperature to high temperature-arid conditions at middle Miocene age when Japan Sea opened and a clockwise rotation of Southwest Japan occurred.
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  • Yujiro Nishimura, Yoshiko Hirota, Daisuke Shiosaki, Nobuyuki Nakahara, ...
    2004 Volume 110 Issue 6 Pages 372-383
    Published: 2004
    Released on J-STAGE: January 07, 2005
    JOURNAL FREE ACCESS
    The Mogi area, northeast of the Nomo Peninsula, Nagasaki Prefecture, exposes the Nagasaki metamorphic rocks of high P/T type. It is divided into two geologic units by the newly detected Mogi thrust. The thrust trends N 22° W and dips 32° W. Its hanging wall, which is named Unit I, consists mostly of pelitic and basic phyllites grading into schists, accompanied by small amounts of metagabbroic and metagranitic rocks. The footwall, which is named Unit II, is typified by a dominance of pelitic and basic spotted schists containing porphyroblastic albite. Mineral assemblages in Unit I indicate the pumpellyiteactinolite facies passing into the chlorite zone of the greenschist facies and/or glaucophane schist facies, whereas those in Unit II indicate biotite zone of the greenschist facies and/or glaucophane schist facies. Carbonaceous material in pelitic metamorphic rocks gives broad spireshaped X-ray powder diffraction peaks with d002 = 3.550d—3.414 Å in Unit I, and sharp peaks with d002 = 3.3619—3.3569 Å indicating fully ordered graphite in Unit II. Remarkably large gaps are revealed between the two units in the mineral assemblages as well as in carbon d002 data. Phengite K-Ar ages from pelitic metamorphic rocks also indicate a large gap between the two units; the ages for 6 separates from Unit I give 214—162 Ma (Late Triassic to Middle Jurassic), and those of 4 separates from Unit II cluster at 87—86 Ma (Late Cretaceous). It is concluded that the Mogi thrust is a northeast extension of the Wakimisaki—Fukabori thrust, and hence Units I and II are assigned to the Suo belt in the Inner Zone and the Sanbagawa belt in the Outer Zone of Southwest Japan respectively. The Mogi and Wakimisaki—Fukabori thrusts are collectively named the Nomo tectonic line. This probably corresponds to a part of the paleo-Median Tectonic Line.
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