烏丸地区深層掘削試料の磁気層序

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  • Magnetostratigraphy of the Karasuma sediment core samples from southeast shore of Lake Biwa
  • カラスマ チク シンソウ クッサク シリョウ ノ ジキソウ ジョ

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The Karasuma deep-drilling core samples recovered from the southeastern shore of Lake Biwa provided the fluvio-lacustrine sedimentary sequence covering the last 1.9 m.y. Although reconnaissance magnetic measurements of the core samples were made in 1990’s, the magnetostratigraphy was not established mainly due to unstable nature of the remanent magnetizations. We made more detailed measurements for the samples from 59 horizons between 370 m and 600 m in depth with alternating field (AF) and thermal demagnetizations. As the previous assessment had suggested, it was difficult to detect high-coercivity or high temperature components clearly. However, our demagnetization results showed presence of the magnetic components mainly at 20–50 mT (AF) or 150–450°C (thermal), among which about half of the samples provided maximum anglular deviation less than 15°. Based on the inclination values of these samples, we suggest that the Brunhes/Matuyama boundary occurs between 433.8 and 451.9 m. It is also suggested that the positive inclination data at 560.8–588.2 m correspond to the Jaramillo Subchron. These magentostratigraphic assignment is fairly consisntent with correlations of the widespread tephra layers found in the Karasuma core samples.

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