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 121, Issue 10
Displaying 1-3 of 3 articles from this issue
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Articles
  • Spatial distribution of pollen assemblages in surface sediments on the Okhotsk Sea basin floor
    Manami Sugaya, Masaaki Okuda, Makoto Okada, Tatsuhiko Sakamoto
    2015 Volume 121 Issue 10 Pages 349-358
    Published: October 15, 2015
    Released on J-STAGE: January 15, 2016
    DOIhttps://doi.org/10.5575/geosoc.2015.0024
    JOURNAL FREE ACCESS
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    The spatial distributions of modern pollen spectra in the Okhotsk basin were analyzed using surface materials obtained from 16 core tops from the Okhotsk Sea floor. The surface pollen is divided into a coastal group and a pelagic group. The coastal group, located within ~100 km from the nearest coast, is dominated by Betula and other deciduous pollen taxa, including pollen of taiga forest, which represents modern vegetation on the neighboring landmass. Assemblages in the pelagic group, in contrast, consist of Pinus and Picea. As these are exotic taxa, their concentrations in pelagic regions are low; these pollen groups, which reflect vegetation in Sakhalin and Siberia, are present only when the distance from land is sufficiently small (<100 km in this context). Pinus becomes dominant when the distance from the coast is greater. These patterns suggest a minimal influence of coastal currents on pollen assemblages in surface sediments in the study region.
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  • The thin-skinned deformations and folding of the upper Awa Group (Neogene) in the Boso Peninsula, Japan
    Akihiro Kokado, Katsushi Sato, Atsushi Yamaji
    2015 Volume 121 Issue 10 Pages 359-372
    Published: October 15, 2015
    Released on J-STAGE: January 15, 2016
    DOIhttps://doi.org/10.5575/geosoc.2015.0020
    JOURNAL FREE ACCESS
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    The folding of Neogene-Quaternary strata on Boso Peninsula, central Japan, has been attributed to differential vertical movements of basement blocks. Previous researchers have suggested that the folding resulted from two-dimensional, thick-skinned deformations along a vertical cross-section perpendicular to the fold axes. To test this hypothesis, we investigated the geologic structures in an area with dimension of 3×3 km2 in the southern part of the peninsula, where the upper Awa Group (upper Miocene-Pliocene fore-arc basin fill) makes E-W trending folds. We found that lateral variations in stratal thicknesses record three-dimensional syn-depositional map-scale movements that are incompatible with the model of two-dimensional fold structures; moreover, outcrop-scale faults also record patterns of three-dimensional deformations. Bedding faults, including a map-scale flat-ramp structure and an outcrop-scale hinterland-dipping duplex, both show NNE vergence, indicating that the strata were subjected to thin-skinned rather than thick-skinned deformation.
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