Abstract:
This paper presents the results of the paleomagnetic and magnetic mineralogical studies carried out along the sequence of the recently discovered oldest Nedubrovo (Early Triassic) rocks of the Moscow Syneclise, exposed in the lower course of the Kichmenga River, where they overlie the Late Permian (Vyatkan) claystone. The Triassic and Permian rock sequence is composed mostly of silty claystone, containing various fauna and flora remains, including those of terrestrial vertebrates. As reported by V. R. Lozovskii et al., the argillaceous rocks contain the products of the chemical reworking and relics of some volcanogenic ash material. The Permian-Triassic transition is recorded in the magnetic properties of the rocks by a high (two- or three-order of magnitude) growth of their magnetic susceptibility and natural remanent magnetization. The most abnormal Permian-Triassic relationship is characteristic of the first layer having a thickness of about 6 m. The data available for the magnetic minerals suggest that characteristic of the Permian claystone is a magnetic goethite-hematite association, and that of the Triassic rocks is a maghemite-magnetite association. The latter is associated genetically with the processes of the low-temperature oxidation of the titanomagnetite. For this reason, the abundance of maghemite and magnetite in the argillaceous rocks of the region is supposed to have been caused by the addition of ash material to the sedimentation basin. The higher values of the magnetic properties of the Early Triassic rocks, compared to the Permian deposits, caused by the presence of maghemite and magnetite, are also characteristic of many other Lower Triassic continental rock sequences in the East of the Russian Platform, and also of the marine rock sequences of South China. In terms of their paleomagnetic properties, the deposits of the Nedubrovo Member show the stable negative polarity of the geomagnetic field.
