中新統瑞浪層群宿洞層のパレオパラドキシア瑞浪釜戸標本を含む 砂岩岩塊の残留磁化および磁気異方性

This paper presents the results of magnetic measurements on the sandstone block containing the “Paleoparadoxiid Mizunami-Kamado specimen” from the Shukunohora Formation of the Miocene Mizunami Group in central Japan. Stepwise demagnetization suggests the presence of both magnetite and greigite, with the latter being probably authigenic minerals formed during early diagenesis. The anisotropy of magnetic susceptibility (AMS) analysis shows a highly variable magnetic fabric with no preferred orientation, suggesting the absence of a consistent grain alignment in the sandstones. This likely reflects disturbances by storms or bioturbation. The sandstone block-mean remanent magnetization direction may be attributed to chemical remanent magnetization (CRM) carried by greigite and, therefore, may not represent the paleofield direction at the time of deposition. However, assuming CRM acquisition during early diagenesis shortly after deposition, the deposition was during a reverse polarity period around 16.5 Ma, potentially either Chron C5Cr (17.154–16.637 Ma), C5Cn.2r (16.532–16.434 Ma), or C5Cn.1r (16.351–16.261 Ma). The deviation of the block-mean remanent magnetization direction from the geocentric axial dipole field suggests a clockwise tectonic rotation (28 ± 20°) for the crust beneath the study area relative to the Asian continent. This rotation coincides with the clockwise rotation of Southwest Japan during the major opening of the Japan Sea, suggesting the “Paleoparadoxiid Mizunami-Kamado specimen” lived amidst this tectonic event.