The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 116, Issue 7
Displaying 1-5 of 5 articles from this issue
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Articles
  • Microscopic observations of varve sediments from Lake Ni-no-Megata and Lake San-no-Megata, Oga Peninsula, NE Japan, with reference to the fallout age of the B-Tm Tephra
    Masaki Kamite, Kazuyoshi Yamada, Megumi Saito-Kato, Mitsuru Okuno, Yos ...
    2010 Volume 116 Issue 7 Pages 349-359
    Published: July 15, 2010
    Released on J-STAGE: November 11, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.349
    JOURNAL FREE ACCESS
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    This paper presents the results of microscopic observations of thin sections of sediment cores from Lakes Ni-no-Megata and San-no-Megata, Oga Peninsula, NE Japan. Micro-sedimentary structures indicate that cycles of light-dark lamina couplets represent biogenic varves, marking annual laminations. We identified 14 complete annual cycles and 1 partial annual cycle between the thin tephra layers of Towada-a (To-a) and Baitoushan-Tomakomai (B-Tm) in sediment cores from the two lakes, suggesting that B-Tm was deposited in AD 929. We also determined that To-a was deposited during early spring, while B-Tm was deposited during one or more eruption events during spring-summer.
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  • The Tanzawa-garnet pumice:
    widely distributed 2.5 Ma tephra in the southern Kanto area, Japan
    Itoko Tamura, Hideo Takagi, Haruo Yamazaki
    2010 Volume 116 Issue 7 Pages 360-373
    Published: July 15, 2010
    Released on J-STAGE: November 11, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.360
    JOURNAL FREE ACCESS
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    We report a distinct garnet-bearing pumiceous tephra in Lower Pleistocene (following the geological time scale in ISC chart 2009) sediments (Kazusa Group) from outcrops in Choshi (Na-G), Kamakura (KGP), and Aikawa-cho (Kanagawa, Mk19), and from a spot core sample (depth: 1217 m) at Koto (Tokyo, KT1217) in the Southern Kanto region. To correlate these garnet-bearing tephras and discuss their possible source, we examined their petrography and performed EPMA and grain-size analyses of constituent garnets, as well as considering their stratigraphic context.
    The garnet-bearing tephras contain sponge- and fiber-shaped glass shards, abundant garnet grains that contain ilmenite inclusions, minor amphibole (orthoamphibole and hornblende), minor biotite, and rare ortho- and clinopyroxenes and cordierite. They also contain rock fragments of green schist and minor granitic rocks. The range of chemical compositions of garnet is XFe=0.50−0.59, XMn=0.18−0.26, XMg=0.14−0.17, and XCa=0.07−0.11, similar to that of garnet phenocrysts from rhyolite (c. 2.5 Ma) in the Nakagawa area, southern Tanzawa Mountains. This garnet composition is markedly different from that of garnet phenocrysts from other Neogene-Quaternary tephras and volcanic rocks in central Japan. The age of the studied garnet tephras is estimated at about 2.5 Ma, based on the stratigraphic horizons at the CN12b-CN12c zone (Okada and Bukry, 1980), the NPD9 zone (Yanagisawa and Akiba, 1998), and the N.21 zone (Yanagisawa, 2006), as well as the magnetostratigraphy of the lower part of the Matuyama Chron.
    The results indicate that the garnet tephras at the four locations are clearly correlatives, representing the newly defined Tanzawa garnet pumice (Tn-GP). The likely source of Tn-GP is located west of Aikawa-cho, in the Tanzawa Mountains, given the westward increase in garnet grain size, the existence of green schist fragments that correlate with the Tanzawa Group, and the existence of ∼2.5 Ma garnet-bearing rhyolite that intrudes into the Tanzawa Group. Tn-GP is likely to play an important role in terms of constraining the P/P boundary in the new geological time scale, the age of the Kurotaki Unconformity, and the subsurface geology of the Kanto Plain.
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  • Vertical structure and emplacement process of the North Unit of the Middle Miocene Kumano Acidic Rocks, Southwest Japan, inferred from the GSJ Kumano-Ichiura drill core
    Takashi Nakajima, Naoji Koizumi, Nobuo Geshi, Teruki Oikawa, Hironao S ...
    2010 Volume 116 Issue 7 Pages 374-387
    Published: July 15, 2010
    Released on J-STAGE: November 11, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.374
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    A 600 m drill core (recovery > 95%) penetrated the Middle Miocene Kumano Acidic Rocks in eastern Kii Peninsula, Southwest Japan. The core is fresh and unaltered, except near the surface, above 40 m below ground level (GL). The core consists of Kumano Granite Porphyry (from GL to 464.25 m deep) and the Owase-Shirahama Pyroclastic Rocks (464.25-600 m deep), with the boundary between these units being an intrusive contact. The Kumano Granite Porphyry is largely homogeneous in rock facies and whole-rock chemical composition, except for the occurrence of a chilled margin at the contact. The porphyry contains phenocrysts of quartz, plagioclase, K-feldspar, orthopyroxene, and biotite. The Owase-Shirahama Pyroclastic Rocks consist of massive crystal tuff. The rocks are largely homogeneous, except for local pumice-bearing parts and tuff beds with horizontal laminations. Many mafic magmatic enclaves and sedimentary-metamorphic xenoliths occur in the Kumano Granite Porphyry and the Owase-Shirahama Pyroclastic Rocks.
    The > 700 m vertical offset of the basement level of the Owase-Shirahama Pyroclastic Rocks between at the drilling site and the nearby outcrops on the ground surfaces, indicates the subsidence of a caldera flow that formed synchronous with or after eruption of the tuffs. The areal distribution of the Kumano Granite Porphyry, including core observations at the intrusive contact, indicate that the porphyry form a sill-like body of > 1,000 m thick, intruding into the caldera-filling Owase-Shirahama Pyroclastic Rocks.
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  • Lithostratigraphy of Upper Guadalupian (Middle Permian) rocks at Chaotian in Sichuan, South China: Secular change in sea level and redox condition of the sedimentary environment
    Masafumi Saitoh, Yukio Isozaki, Jianxin Yao, Zhansheng Ji
    2010 Volume 116 Issue 7 Pages 388-399
    Published: July 15, 2010
    Released on J-STAGE: November 11, 2010
    DOIhttps://doi.org/10.5575/geosoc.116.388
    JOURNAL FREE ACCESS
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    We examined in detail the lithostratigraphy and lithofacies of Middle-Upper Permian shallow-marine limestone (150 m thick) at Chaotian in northern Sichuan, South China, to clarify temporal changes in the sedimentary environment. The main part of the upper Middle Permian Maokou Fm and the lowermost Upper Permian Wujiaping Fm consist of bioclastic (fusuline, algae, and coral) limestone that was probably deposited in the euphotic zone on a continental shelf. In contrast, the uppermost Maokou Fm (11 m thick) is composed of thinly interbedded black shale/chert with abundant radiolarians, which was probably deposited in the dysphotic zone on a continental slope/basin. This change in stratigraphic lithofacies at Chaotian indicates that sea level rose during the latest Guadalupian, but then fell rapidly across the Guadalupian-Lopingian (G-L) boundary. The redox condition of the sedimentary environment also shows a marked change in tandem with sea-level fluctuations. The transgression was probably caused by a local tectonic event that involved basin subsidence in western South China, while the following regression reflects a global sea-level fall across the G-L boundary.
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