Inception of anticline growth near the Omagari Fault, northern Hokkaido, Japan

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  • 北海道北部，大曲断層近傍の背斜成長の開始時期
  • ホッカイドウ ホクブ オオマガリ ダンソウ キンボウ ノ ハイシャ セイチョウ ノ カイシ ジキ

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Abstract

To know the beginning age of the anticline growth near the Omagari Fault in Neogene siliceous rocks, northern Hokkaido, Japan, this study is focused on determining differences in sedimentation rate as the anticline grew during sedimentation, at the sites of two boreholes, which are proximal (HDB-11) and distal (HDB-10), respectively, with respect to the anticlinal axis. We have investigated i) the petrographic and chronologic characteristics of tuff layers within mudstones in terms of several parameters: modal compositions, glass refractive indexes, glass chemical compositions and fission track ages; ii) the depth distribution of the opal-A/opal-CT diagenetic boundary between diatomaceous mudstone and siliceous mudstone; and iii) the effective porosity of the mudstones. Two tuff layers as key beds, HT1 and HT2, were found at HDB-10 and HDB-11, and yielded fission track ages of 2.9 Ma and 2.2 Ma, respectively. The sedimentation rates at HDB-10 and HDB-11 between HT1 and HT2 were estimated to match with the inferred basin trend or to have been nearly equal, based on their fission track ages, the thicknesses and the effective porosity of the mudstones. The depth distribution of the opal-A/opal-CT boundary and the effective porosity indicated that the maximum burial depth of HT2 was significantly shallower at HDB-11 than at HDB-10. Previous studies have found that maximum burial occurred at/before 1.0 Ma, suggesting that the sedimentation rate at HDB-11 was slower than at HDB-10 for the 2.2∼1.0 Ma period overall. We therefore infer that the anticline began to grow between 2.2 and 1.0 Ma. The inception of the anticline growth might be caused by the beginning of the active tectonism in and around this area.

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