[S-GL23] Geochronology and Isotope Geology

[SGL23-P02] Preliminary paleomagnetic results of conglomerates of the Toki Sand and Gravel Formation and the Kurehayama Gravel Formation.

*Kazuo Kawasaki¹, Ken-ichi Yasue¹, Masakazu Niwa², Tatsunori Yokoyama² (1.School of Sustainable Design, University of Toyama, 2.Tono Geoscience Center, Japan Atomic Energy Agency)

Keywords:viscous remanent magnetization, conglomerate, paleomagnetism

The dating of conglomerate formation has been hampered by the lack of suitable fossils or tephra. Here is reported paleomagentic application to date: a) the Toki Sand and Gravel Formation in Gifu prefecture; and, b) the Kurehayama Gravel Formation in the Toyama prefecture. Conglomerate rocks formed during the Brunhes normal epoch are expected to contain a viscous remanent magnetization (VRM) that roughly parallels the present Earth’s magnetic field. VRM is the gradual change of magnetization with time at constant temperature and therefore it can be used as a dating method. Paleomagnetic analyses isolated a VRM by thermal step demagnetization after low temperature demagnetization using liquid nitrogen in the specimens from both sites. Thermomagnetic measurements of the low temperature dependence of the saturation isothermal remanent magnetization indicate that the main magnetic minerals in the conglomerate specimens of both sites are either partially oxidized magnetite or maghemite with some magnetite. The presence of such oxidized magnetite indicates that the observed VRM are likely secondary chemical remanent magnetization or VRM residing oxidized magnetite and therefore the observed VRM is not applicable for dating. Further study is needed to evaluate the efficiency of VRM dating for conglomerates.