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 112, Issue 10
Displaying 1-6 of 6 articles from this issue
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Articles
  • Evaluation of the displacement of the Tsugaru Fault by means of organic maturity of fossil pollen in sediments
    Yoshihiro Ujiié, Takayasu Taniguchi, Masateru Ebina
    2006 Volume 112 Issue 10 Pages 581-593
    Published: 2006
    Released on J-STAGE: February 17, 2007
    DOIhttps://doi.org/10.5575/geosoc.112.581
    JOURNAL FREE ACCESS
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    The Tsugaru Fault, running in N-S direction, in the mountain ridge of the Tsugaru Peninsula, in the northernmost Honshu Island, cut the thick Neogene sediments deeply. In general the Tsugaru Fault is reverse and about 50 km long at least. The fault plane steeply dips west and the west side of the fault was relatively elevated higher than the east side.
    We investigated the Neogene mudstone and sandstone along two transect routes, the Ohkawamezawa and Nishimatazawa, across the Tsugaru Fault. Middle to upper Miocene hard shale, the Kodomari Formation, dips eastward in the west side of the fault, and Plio-Pleistocene sandstone, the Kanita Formation, dips westward in the east side. Organic maturity of 13 samples from the Ohkawamezawa, and 10 samples from the Nishimatazawa transect routes was measured on statistical Thermal Alteration Index (stTAI) and vitrinite reflectance (Ro). The stTAI value should increase and the Ro value should decrease toward the Tsugaru Fault in each side of the fault because the burial depth of sediments decreases toward the fault in both sides. Those phenomena and the effect of faulting are actually recognized only in the values of stTAI, not in those of Ro along the Ohkawamezawa route.
    We employ a simple fault model to calculate the vertical slip, throw of the Tsugaru Fault. Based on the difference in stTAI between both sides of the Tsugaru Fault and geologic characters of the fault observed, we are able to calculate the vertical slip of the fault at 901~1389 m. This estimation is concordant with the previous ones, longer than 1000 m evaluated from the geologic viewpoint (Mimura, 1979) and about 1500 m from the viewpoint of gravity anomaly (Matsuhashi et al., 1989). Our results indicate that organic maturity of sedimentary rocks is a useful tool to study faults as well as diagenesis, unconformity and contact metamorphism.
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  • Distribution of fresh water diatom valves in the surface sediments off the Tokachi-gawa, Teshio-gawa and Ishikari-gawa, Hokkaido, Japan
    Tsumoru Sagayama
    2006 Volume 112 Issue 10 Pages 594-607
    Published: 2006
    Released on J-STAGE: February 17, 2007
    DOIhttps://doi.org/10.5575/geosoc.112.594
    JOURNAL FREE ACCESS
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    Surface marine sediments seaward of the Tokachi-gawa, Teshio-gawa and Ishikari-gawa, the three biggest rivers in Hokkaido, were analysed to determine the distribution of fresh-water diatom valves that were transported to the sea by the rivers. Samples were collected from marine sediments a few cm below the seafloor. Three relationships with the percentage of fresh water valves were examined 1) distance from the river mouth, 2) distance from shore line, and 3) water depth. The results of this investigation show that the percentage of fresh water diatom valves gradually decreases offshore from river mouths, and author believes their distribution being inferred to be controlled by sea water is a significant datum for the current research in coastal area.
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