Origin of Remanent Magnetization and Initial Susceptibility of Certain Red Sandstones

Abstract

Samples of red sandstone, chiefly of the Chugwater formation of Triassic age from Wyoming, have been studied in an attempt to determine the origin of the natural remanent magnetization (NRM) in the rock. Hitherto, this has generally been considered to be the red haematitic coating on the grains and in the interstices of the rocks, but in the samples studied, from which the red material is extracted chemically, it appears that most, if not all, of the NRM is carried by a proportion of the black magnetic particles present. Thermal demagnetization experiments suggest that haematite is the material carrying the NRM. These and other red sandstones, possessing stable and unstable magnetization, have been analysed for ferrous, ferric and titanium oxide content. Initial susceptibility is approximately proportional to ferric or total iron content. Extraction of the red material, as in the experiments on the NRM, confirms that the carrier of the susceptibility is the bulk iron oxide content of the rocks and suggests that the red coating contributes little or nothing to it. Correlation is poor between intensity of NRM and iron content; this might be expected if the NRM is due to only a small proportion of the iron present. Copyright © 1965, Wiley Blackwell. All rights reserved