Paleomagnetostratigraphic Dating of Two Reservoir Sediments of the Sayamaike and Hataoike Ponds in Osaka Prefecture, Japan.

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  • 溜池堆積物の古地磁気年代測定
  • タメイケ タイセキブツ ノ コチジキ ネンダイ ソクテイ

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Abstract

Artificial ponds have been constructed over the past 2, 000 years in the Kinki district of Japan. Accumulated sediments covering the bottoms of two reservoirs, the Sayamaike and Hataoike ponds situated in Osaka Prefecture, were well exposed during the repairing of retention dams. Paleomagnetic studies were carried out with oriented samples directly collected from these bottom sediments. Consequently, high-resolution records of the geomagnetic secular variation were obtained. Based on the correlation with the secular variation reported from archaeomagnetism and sedimentary magnetism, it was discovered that the Sayamaike sediments, which are 630cm thick, record the changes of paleomagnetic directions for the past 500 years and the Hataoike sediments, 320cm thick, recorded those for the past 400 years. The behavior of fine fluctuations is found in both declination and inclination from 1500 to 1800 A. D. The Sayamaike deposits intercalate twelve horizons, designated as S-1-S-12 in ascending order, that indicate sign of liquefaction by earthquake and/or flooding. The results of paleomagnetostratigraphic dating suggest that horizon S-9 at 380cm and S-12 at 220cm date from around 1500-1530 A. D. and 1600-1630 A. D., respectively. The sediments range from the bottom to the S-9 horizon in the Sayamaike pond, around 250cm in thickness, and have magnetization of an almost uniform direction (D=8.2°, I=40.9°, α95=1.2°, k=263.4). The definite direction may suggest that magnetic grains in the sediments were once unlocked and were realigned with the geomagnetic field at the time of the earthquake that caused the liquefaction of the S-9 horizon.

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