The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Displaying 1-10 of 10 articles from this issue
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  • Nagayoshi Katsuta, Sin-iti Sirono, Ayako Umemura, Hirokazu Kawahara, H ...
    2022 Volume 128 Issue 1 Pages 81-86
    Published: January 15, 2022
    Released on J-STAGE: May 21, 2022
    JOURNAL FREE ACCESS

    Fe oxide Liesegang bands have often been observed in sedimentary and igneous rocks, and they are formed during weathering and alteration by water-rock interactions. In this study, micro-X-ray fluorescence (μ-XRF) mapping was used to study the Fe bands in an Fe oxide concretion from the Jurassic Navajo Sandstone in Utah, USA, to estimate the duration of their formation. Most of the peaks in Fe concentration are steeper on the inner side than on the outer side, which indicates a supply of ferrous ions (Fe2+) from outside the concretion. The precipitation of Fe oxide was controlled by pH buffering that resulted from a reaction between acidic water and alkaline pore water that formed through the dissolution of an earlier calcite concretion. The reaction rate within the Fe oxide concretion was estimated from the width of the Fe peaks and the expected diffusion coefficient for Fe through the rock matrix, and it was found to be no more than years to decades-faster than previously estimated. This demonstrates that μ-XRF mapping is a useful technique to extract quantitative information about water-rock interactions from rocks.

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  • Kazuhiko Kano, Yukio Yanagisawa, Mitsuru Okuno, Mitsuhiro Nakagawa, Ki ...
    2022 Volume 128 Issue 1 Pages 43-62
    Published: January 15, 2022
    Released on J-STAGE: April 27, 2022
    JOURNAL FREE ACCESS
    J-STAGE Data

    This paper describes the sedimentological, petrographic, and paleontological features of core samples recovered when drilling a seismic observation well on Shinjima Island, which emerged from the submarine floor of Aira Caldera in 1780 by magma intrusion. The results provide information on post-caldera volcanic activity and environmental change in Aira Caldera. Shortly after its formation at ~30 cal ka BP, Aira Caldera was filled with freshwater, but seawater suddenly replaced the freshwater at ~14.5 cal ka BP as sea level rose after the last glacial maximum. Sakurajima Volcano, located on the southern rim of the caldera, intermittently ejected dacitic tephras into the freshwater caldera lake, and Wakamiko Volcano, located in the northeastern part of the lake, also erupted explosively, releasing sublacustrine rhyolitic density currents over the lake floor. Lahars and floods carried much larger volumes of epiclastic material into the lake. Shortly before the transition from freshwater to seawater, Sakurajima and Wakamiko Volcanoes temporarily became calm and fine particles accumulated in the lake. The explosive eruptions at Wakamiko Volcano resumed at ~13 cal ka BP, culminating in a large eruption that formed Wakamiko Caldera and a thick accumulation of Shinjima Pumice. The explosive eruptions almost ceased with the minor eruption of the Southern Shinjima Pumice immediately after the caldera-forming event. The Southern Shinjima Pumice contains pumice and ash of the Sakurajima Satsuma tephra, indicating that the largest explosive eruption from Sakurajima Volcano occurred during the period between the eruptions of the Shinjima Pumice and the Southern Shinjima Pumice. Since the caldera-forming eruption, Wakamiko Caldera has become quiescent while Sakurajima Volcano has intermittently erupted, producing fallout tephras and lava flows. The Sakurajima tephras and volcaniclastic materials reworked from surrounding areas have accumulated on the submarine floor of Aira Caldera.

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Articles
  • Junko Komatsubara, Ikuo Cho, Kentaro Sakata, Tsutomu Nakazawa
    2022 Volume 128 Issue 1 Pages 29-42
    Published: January 15, 2022
    Released on J-STAGE: March 10, 2022
    JOURNAL FREE ACCESS

    The average S-wave velocity and peak frequency of the microtremor H/V spectrum, or the resonance frequency of the ground, were estimated at 16 sites in the Tokyo Lowland along the coast of inner part of Tokyo Bay, central Japan. The results indicate that the resonance frequencies become lower with an increase in thickness of the post-Last Glacial Maximum (LGM) deposits. The post-LGM deposits are divided into the lower Nanagochi Formation and the upper Yurakucho Formation in the Tokyo Lowland. The depth of the S-wave velocity discontinuity, which determines the resonance frequency of the ground, was calculated by applying the quarter-wavelength law. At 7 of the 13 microtremor observation points examined (i.e., 3 points on a buried flat surface are excluded from the examination), the S-wave velocity discontinuity is located within alternating layers of sand and mud in the Nanagochi Formation, even though the boundary between the Nanagochi and Yurakucho formations has conventionally been considered the most significant boundary in terms of physical properties in the post-LGM deposits. This discrepancy is perhaps caused by the strong influence of either the lithologic boundaries in the alternatin layers of sand and mud in the upper part of the Nanagochi Formation, or the more conspicuous boundary at the top of the basal gravels of the Nanagochi Formation, or both. The variations in the peak frequency of the H/V spectrum are caused by the heterogeneity of meandering river and estuary deposits in the Nanagochi Formation.

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  • Yoshihiro Kase, Gentaro Kawakami, Hiromichi Koyasu, Ryo Takahashi, Tsu ...
    2022 Volume 128 Issue 1 Pages 7-26
    Published: January 15, 2022
    Released on J-STAGE: February 15, 2022
    JOURNAL FREE ACCESS
    J-STAGE Data

    Geological records of tsunamis along the Tsugaru Strait in southwestern Hokkaido, Japan, were investigated. Event deposits were identified in lowland peat at four survey sites located 100-450 m from the present coastline and 2.2-6.6 m above the present sea level. The event deposits are several centimeters thick and consist of well-sorted fine- to medium-grained sand. Based on radiocarbon dating and tephrochronology, the sedimentary ages of the event deposits were determined to be 589-516, 734-670, 1656-1538, 1745-1639, 2401-2265, and 2771-2618 cal yBP. Comparison of the clastic composition and roundness of the event deposits with those of nearby beach and river sands revealed that most of the provenance of the event deposits are beach sands. Whereas, some of the event deposits were sourced from volcanic-ash sands, which were distinct from both beach and river sands. The chemical compositions of volcanic glasses and mineral grains indicate that the volcanic ash did not originate from any known Quaternary tephra in this region. Considering the source of the event deposits, as well as the inundation distance from the coastline and the infrequency of events, the event deposits were likely formed by tsunamis. Volcanic-ash sands may have been widespread in the seafloor beyond the foreshore and were transported by tsunamis. From the sedimentary ages and depositional rates of event deposits and peat, the youngest event deposits were formed in the 13-15th century, and may be correlated with a tsunami that reached the eastern coast of Hokkaido in the 13-14th century or the AD 1454 Kyotoku tsunami. Neither the well-known 17th century tsunami that was triggered along the Pacific coast of eastern Hokkaido nor the AD 1611 Keicho-sanriku tsunami appear to have left deposits in this area. These results can be used to determinate accurately the source area of 17th century tsunamis.

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Letter
  • Takayuki Uchino, Yasuyuki Banno
    2022 Volume 128 Issue 1 Pages 1-6
    Published: January 10, 2022
    Released on J-STAGE: February 01, 2022
    JOURNAL FREE ACCESS

    An exotic block of garnet-bearing schist has been found at the boundary between the Early Triassic Takinosawa Unit of the Nedamo Belt and the Early Jurassic Nakatsugawa Complex of the North Kitakami Belt, NE Japan. This is only the second discovery of exotic schists in the Nedamo Belt; the previously identified schists occur at the boundary between the Takinosawa and early Carboniferous Tsunatori units of the Nedamo Belt. The garnet-bearing schist is characterized by a mineral assemblage of garnet + phengite + epidote + albite + quartz + titanite. Phengite with Si of 6.51-6.66 atoms per formula unit (O = 22) and the mineral assemblage suggest that the schist underwent high-P/T metamorphism prior to emplacement into its current position. The schist yields a phengite K-Ar age of ca. 290 Ma (late Paleozoic), suggesting a correlation with high-P/T schist of the Yamagami Metamorphic Rocks from the Motai-Matsugadaira Belt. This finding supports the extension of late Paleozoic high-P/T metamorphic rocks (i.e., the Renge Belt of SW Japan) to NE Japan.

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