火山砕せつ物の地磁気層序とその連続性

Abstract

Paleomagnetic study and potassium-argon dating of igneous rocks have led to an establishment of a geomagnetic polarity time scale for the past 4.5m.y., Late Cenozoic polarity changes have been recognized throughout the world in deep-sea sediments and outcropping sedimentary sequences. However it has been suggested that the establishment of reliable magnetostratigraphy is difficult in some cases because of post-depositional remagnetization resulting from chemical precipitation and the acquisition of VRM.Sometimes experimental techniques have not allowed distinction between the original geomagnetic record from the secondary magnetization. It is thought therefore, that the most convincing way to demonstrate the reliability of magnetostratigraphy is to reproduce the results in separate sections showing different lithology and sedimentation rates.In accordance with this concept, a field stability test of remanent magnetism of the pyroclastic sediments deposited in and around the Aizu Basin, Fukushima Prefecture, was conducted. The following conclusions can be drawn concerning the reliability of magnetostratigraphy:(1) The DRM of pyroclastic materials in the Nanaorizaka Formation is as stable as the TRM of welded tuffs considered to be their correlatives in the Seaburiyama and Shirakawa Formations.(2) Stability of the DRM is also supported by the folding test made on the key tuffaceous bed within the Nanaorizaka Formation.(3) The reliability of the magnetostratigraphy of the Fujitoge Formation was demonstrated by reconfirmation of the same reversed polarity magnetozone in four separate sections with differing sedimentation rates and lithologies.