The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Current issue
Showing 1-4 articles out of 4 articles from the selected issue
Articles
  • Ryo Nakanishi, Satoshi Okamura
    2019 Volume 125 Issue 12 Pages 835-851
    Published: December 15, 2019
    Released: March 26, 2020
    JOURNALS FREE ACCESS

    Coastal sediments in Uchiura Bay and along the Iburi coast, on the western part of the Pacific coast in Hokkaido, contain organic sediments intercalated with tsunami-derived sediments from a 17th-century tsunami event. The event layers show sedimentary features characteristic of a tsunami origin, including thinning of layers, sediment fining, and decreasing heavy mineral content in an inland direction. From Uchiura Bay to the western Iburi coast, the event layers are directly overlain by the 1640 Komagatake tephra (Ko-d). From the eastern part to the central part of the Iburi coast, the event layers deposit below the 1663 Usu tephra (Us-b), but there is almost no gap between them because there is only a very thin humus layer. We infer that the event layers record the tsunami caused by the 1640 Komagatake eruption, because this eruption produced both a large tsunami and a large amount of tephra (Ko-d). Our numerical simulation based on the distribution of the event layers indicates that more than 1.2 km3 of debris formed an avalanche into the sea, causing a tsunami of Mt 7.9-8.2. The estimated run-up heights showed good agreement with the observed run-up heights from Uchiura Bay to the Shiraoi area of the Iburi coast.

    Download PDF (7073K)
  • Yoshitaka Kakuwa, Naizhong Zhang, Ryo Matsumoto, Hitoshi Tomaru, Naoto ...
    2019 Volume 125 Issue 12 Pages 853-865
    Published: December 15, 2019
    Released: March 26, 2020
    JOURNALS FREE ACCESS

    The low-temperature carbonate mineral ikaite was recovered from sediment cores from the Sado West area, in the eastern margin of the Japan Sea. The ikaite-bearing sediments consist of dark-colored, bioturbated or burrow-mottled mud with sand and silt laminae. There are rare occurrences of ikaite within mud containing thin, parallel laminations indicative of anoxic sedimentary environments. Ikaite occurs either as veins of granular aggregates or as large crystals, some of which exceed 10 cm in length. The nature of the ikaite occurrences indicate that it is unlikely that ikaite formed directly on the seafloor. The ikaite deposits are found at depths shallower than 10 meters below the sea floor (mbsf), but pore-water analysis suggests that the ikaite crystals formed within the sulfate-methane transition (SMT) zone, likely at 2-5 mbsf. The ikaite stable carbon isotopic ratios are low, −33 to −34 per mil, indicating that carbon derived from microbial oxidation of methane gas was involved in ikaite formation, consistent with the occurrence of ikaite within the SMT. The most recent formation of ikaite is calculated to be more than 9,700 years ago. All ikaite occurrences are in cores that do not contain methane hydrates. Hydrate-bearing cores are usually associated with calcite nodules. Further investigation is required to determine the controlling factors in the formation of either ikaite or ordinary calcite. Ikaite becomes unstable at depths greater than 70 mbsf as the methane flux or the temperature increases during burial. Large crystals of ikaite are replaced by stable calcite to form Gennou Ishi.

    Download PDF (7262K)
  • Toshiki Haji, Atsushi Yamaji, Sota Niki, Takafumi Hirata
    2019 Volume 125 Issue 12 Pages 867-785
    Published: December 15, 2019
    Released: March 26, 2020
    JOURNALS FREE ACCESS

    The lower Miocene Yoka Formation in southwest Japan is composed of terrestrial volcaniclastic rocks, and contains no index fossils. The basal part of the formation has yielded zircon U-Pb and fission-track ages of ~21.5 Ma. The upper part of the Toyooka Formation, which unconformably overlies the Yoka Formation, correlated with 17-16.5 Ma by index fossils and zircon ages. In this 5 Myr. long age gap, marine transgression and paleomagnetic rotation occurred in this area. We collected a felsic lapilli tuff sample from the middle part of the Yoka Formation, and the U-Pb ages of zircons were measured using a laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The weighted average of concordant 238U/206Pb age of 19.38±0.23 Ma(95% conf., MSWD = 9.4)was obtained from the zircons excluding an outlier age from one grain. This zircon U-Pb age indicates that the transgression and paleomagnetic rotation occurred rapidly within ~2.5 Myr in this area.

    Download PDF (2950K)
Report
  • Tomio Nakagawa, Hiroyoshi Sano, Sachiko Agematsu, Katsuo Sashida, Yuki ...
    2019 Volume 125 Issue 12 Pages 877-884
    Published: December 15, 2019
    Released: March 26, 2020
    JOURNALS FREE ACCESS

    This paper describes Upper Triassic limestone clasts of the polymictic limestone conglomerate (Imogadaira Limestone) of the Mino Belt in the Nanjo Mountains, Fukui Prefecture, central Japan. The Imogadaira Limestone occurs as an isolated block in the Middle Jurassic mélange unit in the upper reaches of the Imogadaira River, Nanjo Mountains. The formation exhibits a prevailing conglomerate fabric, characterized by dense packing of limestone clasts within a sparse matrix. The Imogadaira Limestone has a polymictic clast association, dominated by Permian limestones with lime mudstone and limestone clasts containing thin-shelled bivalves. Neither terrigenous clastic grains nor rock fragments occur in the Imogadaira Limestone. Late Triassic conodonts and Late Triassic and Early Jurassic radiolarians were extracted from whole-rock samples of the Imogadaira Limestone. Further examination revealed that the Late Triassic conodonts and radiolarians occurred within limestone clasts containing thin-shelled bivalves.

    Download PDF (4501K)
feedback
Top