The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 120, Issue 7
Displaying 1-3 of 3 articles from this issue
Articles
  • Masayuki Utsunomiya, Chihiro Nagahama, Robert G. Jenkins, Atsushi Noza ...
    2014 Volume 120 Issue 7 Pages 221-231
    Published: July 15, 2014
    Released on J-STAGE: November 12, 2014
    JOURNAL FREE ACCESS
    A shell-concentrated sandstone bed (50-53 cm thick), containing sporadic vesicomyid and thyasirid shells up to 10 cm in diameter, occurs in the Lower Pleistocene Nojima Formation, which is a forearc basin fill on the Miura Peninsula, Pacific side of central Japan. The sandstone bed consists of three units (in ascending order): Unit 1, a cm-thick reverse-graded, coarse-grained to pebbly coarse-grained sandstone; Unit 2, a 25-cm-thick normally graded, pebbly coarse-grained to fine-grained sandstone; and Unit 3, a 10-cm-thick parallellaminated, fine-grained sandstone. Units 1 and 2 contain abundant small molluscan shells, commonly with a maximum length of less than 1 cm and including a wide variety of shallow- to deep-water species, whereas molluscan shells are rare in Unit 3. Vesicomyid and thyasirid shells occur mainly in Unit 2, and rarely in Unit 3. The molluscan shells throughout bed are commonly fragmented and abraded.
    We measured the shell orientations of bivalves and gastropods in units 1 and 2, excluding shell fragments. In Unit 1, the axes of the gastropod shells and the commissure planes of the bivalve shells are randomly oriented, but those in Unit 2 commonly dip southward.
    We interpreted the sandstone bed as having been deposited by both a debris flow (Unit 1) and a turbidity current (Units 2 and 3) occurring in sequence during a single gravity flow event with a north-ward flow direction. This sediment gravity flow originated in shallow-water sediments containing shallow-water molluscan shells and subsequently entrained mollusks living in a wide variety of environments, including a cold seep site where vesicomyids and thyasirids flourished. These results indicate the existence of active cold seepage not at the study exposure, but south of the exposure, at the time of the sediment gravity flow event.
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  • Takahiro Yamamoto, Yoshihisa Kawanabe
    2014 Volume 120 Issue 7 Pages 233-245
    Published: July 15, 2014
    Released on J-STAGE: November 12, 2014
    JOURNAL FREE ACCESS
    Supplementary material
    The Izu-Oshima 2013 lahar occurred on the western slope of Izu-Oshima volcano in response to heavy rain brought by the 26th Typhoon (WIPHA) on 16 October 2013. The lahar damaged Motomachi village, resulting in many fatalities. We immediately carried out field surveys and compared the grain-size distributions of this lahar deposit with those of various reference lahars, including the Unzen 1993, Pinatubo 1991-1995, Bandai 1888, Izu-Oshima S2-c, Adatara-Sukawa L1, and Numazawa-Numazawako lahars. Based on the grain-size analysis, we divided these deposits into three types: cohesive mud flow, non-cohesive debris flow, and hyperconcentrated flood flow. The Izu-Oshima 2013 lahar deposit consists of moderately sorted, coarse- to medium-grained sand with pebbles and well-sorted fine- to very-fine-grained sand. The bi- and uni-modal grain-size distributions of this deposit suggest that the lahar was emplaced as a hyperconcentrated flood flow. This lahar was generated by a shallowseated slope failure within ash fall deposits covering the Y5-stage collapse wall, and it cascaded turbulently down to Motomachi village at high speed.
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Report
  • Mitsuo Shimazu, Hideki Nishita, Hiromi Konishi, Yoshimi Takeshita
    2014 Volume 120 Issue 7 Pages 247-253
    Published: July 15, 2014
    Released on J-STAGE: November 12, 2014
    JOURNAL FREE ACCESS
    Asphalt and asphaltite were found in the Yashiroda oil field where crude oil was produced between 1869 and 1925. This is a report on the first finding of asphaltite in Japan, covering the asphalt mass of heavy crude oil origin that flowed from a drilled well. The asphaltite specimen is black with brilliant luster and conchoidal fracture, and has a specific gravity of 1.17. X-ray diffraction analysis confirmed it as amorphous as evident from a broad peak at 2θ = 18°-25°. Biomarker analysis by GC-MS showed that the asphalt and asphaltite specimens are both of marine origin, the same as the crude oil bitumen collected from the Niitsu oil field. Many excavated asphalt remains at the Osawayachi ruins (Jomon-Heian to Kamakura periods) might have originated from the Yashiroda oil field, which is situated 600 m east of the excavation area.
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