Timing of remanent magnetization acquisition in red beds: A case study from a syn-folding sedimentary basin

Red beds have been recognized as a useful source of ancient geomagnetic field. However, the timing of remanence acquisition in red beds is still incompletely understood. Fold tests may be used for an estimation of the timing of red bed remanence acquisition, but an essential ability of this kind of test is to conclude whether magnetizations are acquired before or after folding. Even a positive fold test does not necessarily represent a primary nature of magnetization. Because the relationship between the timing of fold growth and sedimentation is well understood for a small, syn-folding sedimentary basin, we select such a sedimentary basin as a focus of study. A case study example examining the timing of red bed remanence acquisition based on fold tests is described in this paper. It shows that the observed remanence directions carried by hematite are given a syn-folding nature. This syn-folding nature is more observable in the lower part of the sedimentary sequence that was under the more intense folding during its sedimentation than the upper part. A pure depositional remanent magnetization when beds were horizontal is not applicable to this case, but a post-depositional remanent magnetization, with possible existence of an initial slope of the deposition surface, is considered to have been imparted prior to complete lithification. Microscopic observations showing the presence of hematite as sedimentary particles can be used for an independent estimation supporting the detrital origin. Chemical compositions of the hematite grains agree well with the maximum laboratory unblocking temperatures of the samples, and we draw the conclusion that these grains are the carrier of the detrital magnetization.